Braking system for electromagnetic motors

ABSTRACT

A method for controlling a braking system of an electromagnetic motor, the electromagnetic motor having a moveable output shaft, comprising the steps of: receiving a velocity signal and/or an acceleration signal based on movement of the output shaft, said velocity signal and/or acceleration signal having a respective frequency spectrum; identifying an event from the velocity and/or the acceleration signal using the respective frequency spectrum, wherein said event corresponds to an uncontrolled movement of the output shaft and has a characteristic frequency spectrum.

TECHNICAL FIELD

The present invention relates to emergency braking systems and methodsof use thereof, particularly those for electromagnetic motors for use intest apparatus.

BACKGROUND

Both common forms of motor used in test apparatus, servo-hydraulic andelectromagnetic, comprise output shafts with large masses. Should afault occur within the apparatus, the large mass can accelerate andcause damage to both the apparatus and any persons in the vicinity.Braking systems are therefore designed to prevent undesired movements ofthe output shaft.

Servo-hydraulic systems may comprise mechanical brakes whereby themovement of the output shaft is managed by gripping or releasing theoutput shaft and/or the movement of the output shaft can be managed bycontrolling the flow of oil in the hydraulic cylinder.

Electromagnetic motors are not operated by the same principles as aservo-hydraulic motor. Instead, an output shaft comprising magneticmaterial is positioned within a coil assembly, alternatively, movingcoil electromagnetic motors mount the coil assembly to the output shaftand positioned within a housing comprising magnetic material. The coilassembly comprises a plurality of separate coil loops, an electriccurrent signal applied through the coil assembly induces a magneticfield which interacts with the magnetic material's magnetic field andtherefore a force on the output shaft is generated dependent on themagnitude and direction of the applied current. This force can be usedto accelerate or decelerate a moving output shaft, additionally it canbe used to counteract gravitational force acting on the output shaftwhen the test apparatus is in a vertical orientation or any orientationother than horizontal. In a non-horizontal orientation, it will beappreciated that in the absence of electrical power, the output shaft isfree to move under the effect of gravity. Therefore, a mechanical brakemust be actuated to prevent undesired movement of the output shaft suchas when the motor is switched off. An additional means for braking amoving output shaft of an electromagnetic motor is throughelectromagnetic induction whereby motion of the output shaft within thecoil assembly induces eddy currents in the output shaft so as togenerate an electromagnetic braking force between the output shaft andthe coils otherwise known as an e-brake.

While there is no current standard for compliance which dictates howquickly an output shaft with undesired motion must be arrested, it isdesirable that motion be arrested prior to the output shaft travelling 2mm or less and/or in the case of rotary motors: 360 degrees. Due to thelarge combined mass of the output shaft, it is possible for the outputshaft to travel the length of its travel within a few milliseconds whenacting only under gravity. Thus, the response time of the braking systemin engaging either the mechanical brake, or reversing the currentapplied to the coil assembly or applying an e-brake is required to bevery short to prevent 2 mm or of travel.

Several known methods for controlling the actuation of the differentbraking methods involve monitoring the velocity of the output shaft. Inan ideal situation, a perfect Safely Limited Speed (SLS) mechanism isused to pre-emptively prevent the output shaft from achieving speedsgreater than a predetermined threshold—typically 10 mm/s or 30 deg/s forrotary movement. Unfortunately, current SLS mechanisms are unable toperform this desired function; they operate by monitoring the velocityand raising a fault condition when the threshold is exceeded. Therefore,the velocity of the output shaft has already exceeded the threshold oncethe fault condition has been raised—the SLS does not pre-emptively limitthe speed to avoid exceeding the threshold, it reacts once the thresholdis exceeded. This is equivalent to a Safe Speed Monitor (SSM) thatmonitors velocity and generates a safety and/or fault signal when thevelocity is below or above a predetermined threshold, in conjunctionwith a Safe Torque Off (STO) mechanism that shorts the windings on thecoil assembly producing a magnetic braking effect. Similarly, monitoringof the velocity can be extended to monitoring of the acceleration of theoutput shaft wherein the STO is engaged when a predeterminedacceleration threshold is exceeded—typically 30 mm/s² or 90 deg/s² forrotary movement.

Several problems exist in relation to the above braking system that usesan SSM in conjunction with an STO to prevent undesired movement. Thereare scenarios that exist in which the threshold for either velocity oracceleration can be exceeded but a fault has not occurred. These aretermed false-triggering events or nuisance tripping. In these scenarios,the braking system engages and the apparatus is made safe until aninspection can be carried out. The frequency at which thesefalse-triggering events occur results in a sizable proportion ofoperators for test apparatus switching off the emergency braking system;a practice which can lead not only to apparatus damage but also injuryto operators.

Examples of scenarios where the threshold for either velocity oracceleration is exceeded but a fault has not occurred include (but arenot limited to):

1. Each time the mechanical brake is released, an impulse is experiencedby the output shaft resulting in large acceleration and velocity.

2. Working in the environment with the test apparatus, the apparatus canbe knocked accidentally, resulting in large acceleration and velocity.

3. Adjusting the load string with tools can result in impulses thatproduce large acceleration and velocity.

4. Operating the specimen holding grips can result in impulses thatproduce large acceleration and velocity.

5. When operating in a set-up mode, in which the current applied to themotor is strictly limited, ‘motor cogging’ can result in the velocityand acceleration thresholds being exceeded.

There is therefore a need to develop an emergency system that has a highaccuracy in differentiating false-triggering events from actuallyundesired output shaft motion.

Conventional means for arresting the motion of an uncontrolled outputshaft is the use of an electrical brake: by connecting each of theplurality of the separate coil loops of the coil assembly to each other.To do this, a mechanical device is used; a delay between the instructionto apply the e-brake and the mechanical device enacting the brakeresults in the time between identifying a failure event and the outputshaft being arrested being of the order 10 milliseconds. A delay of thisduration is not sufficient to bring the output shaft to rest within 2 mmof travel and as such a new device is required to decrease the distanceof travel by reducing the delay time.

Test apparatus must abide by certain safety standards, one such standardinvolves the lifetime of the mechanical brake pads used in eachmechanical brake. It is therefore desired to produce a means formonitoring the lifetime of brake pads fitted to a mechanical brake.

SUMMARY

A first aspect of the present invention relates to a method forcontrolling the braking system of an electromagnetic motor, theelectromagnetic motor having a moveable output shaft, comprising thesteps of: receiving a velocity signal and/or an acceleration signalbased on movement of the output shaft, said velocity signal and/oracceleration signal having a respective frequency spectrum; identifyingan event from the velocity and/or the acceleration signal using therespective frequency spectrum, wherein said event corresponds to anuncontrolled movement of the output shaft and has a characteristicfrequency spectrum.

Identifying the event comprises filtering the velocity and/oracceleration signal to attenuate one or more frequency components of thefrequency spectrum; the one or more frequency components attenuated bythe filter can represent a part or a whole of a frequency profile of theuncontrolled movement of the output shaft that does not pose a risk toan operator. As such, the applicant gives a way of distinguishingbetween previously indistinguishable events.

To reduce the delay between triggering actuation of the electric brakeand the actual actuation, the second aspect of the present inventionproposes a device comprising: a coil assembly circuit comprising aplurality of separate coil loops configured to cause movement of anoutput shaft of the motor while electrical power is applied; a switchingdevice configured to form an electrical connection between the pluralityof separate coil loops of the coil assembly circuit such that movementof the output shaft is arrested; and an opto-isolator for actuating theswitching device. Said device also has the benefit of electricallyisolating high voltage circuits from low voltage circuits thereforeproviding a means for protecting components from damage.

The force acting on the output shaft and therefore, the movement of theoutput shaft with respect to the coil assembly is dependent on themagnitude and direction of the current applied to the coils in the coilassembly. When the testing machine is turned off, the output shaft canbe in any position since the mechanical brake is engaged with the outputshaft when the power is turned off. During the initial start-up of thetesting machine, i.e. when the testing machine is switched on, there isa sudden jolt as the mechanical brake disengages from the output shaft,i.e. the brake shoe disengages from the output shaft. Such a sudden joltcan be considered as a failure event discussed above with respect to thefirst aspect of the present invention and detected as an impulse thatwould trigger a fault condition.

To mitigate the movement of the output shaft when the mechanical brakeis released from engagement with the output shaft, e.g. during initialstart-up of the testing machine, the third aspect of the presentinvention suggests a method of correlating the position of the outputshaft with a current applied to a coil assembly necessary to resistmovement of the output shaft. Preferably, the method comprising thesteps of: determining the position of the output shaft; determining acurrent based on the position of the output shaft that when applied inthe coil assembly induces a force on the output shaft to prevent motionof the output shaft when the mechanical brake is released and, applyingthe current to the coil assembly. Said method can also comprise using alook-up table to determine the current applied to the coil assemblybased on the position of the output shaft. To generate said look uptable, the current in the coil assembly is sampled at fixed intervals ofthe axial displacement of the output shaft. Preferably the applicantalso proposes the method of: determining a first current needed to holdthe output shaft in a first position; determining, a second currentneeded to hold the output shaft in a second position; storing the firstand second positions together with the first current and second currentin the look-up table.

The present invention provides, in its fourth aspect, a mechanical brakedesign that increases the speed of engagement of the brake with theoutput shaft. Said brake comprises a pivotally mounted plate having aspace for receiving the output shaft of the motor; an electricallyoperated holding device contacting a free end of the plate and arrangedto hold the plate in a condition to permit movement of the output shaftand to permit the plate to pivot to a jamming position; wherein theelectrically operated holding device comprises a solenoid to control themovement of the plate. The braking action of the mechanical brake of thepresent invention is self-energising which means that the weight of theoutput shaft acting downwards under gravity generates the braking forcenecessary to hold the output shaft in position. The solenoid can be alinear solenoid to provide an advanced actuation mechanism over thatknown in the prior art. Furthermore, the mechanical brake can comprise aresilient member arranged to bias the plate towards the jammingposition. Optionally, the mechanical brake is fabricated as a modularcomponent.

The present invention provides, in its fifth aspect, a method formonitoring the performance of a mechanical brake for a linearelectromagnetic motor, the linear motor having a linearly moveableoutput shaft, comprising monitoring travel of the output shaft over theduration of actuation of the mechanical brake and comparing said travelwith a predetermined travel threshold. Said method provides a means tomonitor reliability, ensure proper functioning and increase safety ofthe linear motor. A user can be alerted to the degree of travel so thatthey can take appropriate actions.

In some situations, the mechanical brake of the fourth aspect of thepresent invention may fail to engage for a variety of reasons. Forexample, the solenoid of the mechanical brake may fail or is jammed dueto a fault in the electrical switching which fails to remove the powerto the solenoid holding the elongated plate in the released conditionand thereby, preventing the elongated plate to engage with the outputshaft or the brake shoe of the mechanical brake fails to engage with theoutput shaft, e.g. worn out. In an event of failure of the mechanicalbrake and to prevent the output shaft from falling and creating acrushing force, in a sixth aspect of the present invention, the presentinvention provides a redundant mechanical brake. Preferably, themechanical brake of the present invention comprises a primary mechanicalbrake and a secondary (redundant) mechanical brake. The mechanical brakediscussed above being the primary mechanical brake and wherein themechanical brake further comprises a secondary mechanical brake. Thesecondary mechanical brake behaves similarly to the primary mechanicalbrake of the present invention, i.e. comprising a pivotally mountedelongated plate having a space for receiving the output shaft and whichengages with the output shaft when in the brake “on” condition and anelectrically operated holding device to control the movement of apivotally mounted second plate. To accommodate both the primarymechanical brake and the secondary mechanical brake, the primarymechanical brake may be rotationally offset to the secondary mechanicalbrake about the longitudinal axis of the output shaft such that theirrespective plates pivot about non-parallel axes. The secondarymechanical brake cooperates with the primary mechanical brake to providethe functional features of the mechanical brake discussed above.Preferably, the primary mechanical brake comprises a pivotally mountedfirst plate and the secondary mechanical brake comprises a pivotallymounted second plate, each of the pivotally mounted first and secondplate comprises a space that are co-axial for receiving the output shaftof the motor, wherein the secondary mechanical brake cooperates with theprimary mechanical brake to provide a stop spaced apart from a lowersurface of the second plate and arranged to contact the lower surface ofthe second plate when a force acting on the second plate by the outputshaft exceeds a threshold. Preferably, the electrically operated holdingdevice (e.g. solenoid) of the secondary mechanical brake is coupled tothe pivotally mounted first plate of the primary mechanical brake suchthat the electrically operated holding device of the secondarymechanical brake rides on the pivotally mounted first plate. Morepreferably, the pivotally mounted second plate is pivotally mounted on aresiliently loaded (or supported) fulcrum which is capable of movementwhen the force acting on the second plate by the output shaft exceedsthe threshold such that the second plate pivots about the stop andovercomes the biasing force of the resilient member, wherein theresiliently loaded (or supported) fulcrum is provided by the cooperationof the secondary mechanical brake with the primary mechanical brake.

Preferably, in use, the pivotally mounted first plate of the firstmechanical brake and the pivotally mounted second plate of the secondmechanical brake are arranged to pivot independently in sequence orsimultaneously. The first mechanical brake and the second mechanicalbrake act in parallel to arrest the movement of the output shaft. Innormal braking, both the pivotally mounted first plate and the pivotallymounted second plate pivot to engage with the output shaft, morespecifically the spacing for receiving the output shaft engages with theoutput shaft. There are two conditions of engagement of the mechanicalbrake with the output shaft. The first is “arming” of the mechanicalbrake and represents the condition whereby the mechanical brake is notor not entirely loaded by the weight of the output shaft and therefore,the elongated plated, more specifically the spacing for receiving theoutput shaft is not jammed against the output shaft. The secondcondition is full braking and occurs when the mechanical brake becomesloaded as the weight of the output shaft causes the spacing forreceiving the output shaft to jam further against the output shaft totake up the load of the output shaft as the plate tilts. Optionally, theload to hold the output shaft is shared between the primary mechanicalbrake and the secondary mechanical brake. Preferably, the primarymechanical brake takes more of the share of the load of the output shaftthan the secondary mechanical brake in the normal braking condition.Preferably, in the normal braking condition, the primary mechanicalbrake is loaded, i.e. takes the load of the output shaft and thesecondary mechanical brake is armed. By arming the secondary mechanicalbrake whilst the primary mechanical brake shares a greater portion ofthe load of the output shaft, preserves the brake shoe of the secondarymechanical brake and therefore, there is less likelihood that the brakeshoe of the secondary mechanical brake will wear out before the brakeshoe of the primary mechanical brake. Thus, should the primarymechanical brake fail, the brake shoe of the secondary mechanical brakewould still be in a condition to take up the load of the output shaftwithout excessive slipping.

Preferably, the pivotally mounted first plate of the primary mechanicalbrake pivots at a greater angle with respect to the horizontal axis thanthe pivotally mounted second plate of the secondary mechanical brake inthe normal braking condition. By allowing the pivotally mounted secondplate to pivot further than the pivotally mounted first plate, thepivotally mounted first plate will engage with the output shaft afraction of a second earlier than the pivotally mounted second platewith the resultant effect that the primary mechanical brake will share agreater portion or all of the load of the output shaft than thesecondary mechanical brake which will be in the armed condition. Thesecondary mechanical brake will become more loaded, .i.e. out of thearmed condition, when the primary mechanical starts to fail. The lesserthe amount of load of the output shaft that is supported by the primarymechanical brake, i.e. the lesser it engages with the output shaft, thegreater the share of the load of the output shaft is taken up by thesecondary mechanical brake (and vice-versa) and thereby, providingadditional safety.

Each aspect of the present invention can be combined with any otheraspect, as can each option within each aspect; unless they arespecifically taught as alternatives. The different aspects of theinvention individually or in any combination provide an improved testapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred features and aspects of the present invention will beapparent from the claims and the following illustrative description madewith reference to the accompanying drawings in which:

FIG. 1 is a flow chart representation of an electromagnetic motorcontrol system known in the prior art.

FIG. 2 is a flow chart representation of an electromagnetic motorcontrol system according to a first aspect of the present invention.

FIG. 3A is plot of the frequency response of the FIR filter on the firstchannel according to an option of the first aspect of the presentinvention.

FIG. 3B is plot of the frequency response of the FIR filter on thesecond channel according to an option of the first aspect of the presentinvention.

FIG. 4 is a flow chart representation of analysis of motion of theoutput shaft according to an option of the first aspect of the presentinvention.

FIGS. 5A and 5B are illustrative graphic plots of the raw velocitysignal and filtered velocity signal against time respectively accordingto an option of the first aspect of the present invention.

FIGS. 5C and 5D are illustrative graphic plots of the raw accelerationsignal and filtered acceleration signal against time respectivelyaccording to an option of the first aspect of the present invention.

FIG. 6A is a circuit diagram of a mechanical relay switch known in theprior art.

FIGS. 6B and 6C are circuit diagrams of Solid-State-Relay circuitswitches according to a second aspect of the present invention.

FIG. 7 is a illustrative graphic plot of the current required to holdthe output shaft stationary against the action of gravity vs theposition of the output shaft according to a third aspect of the presentinvention.

FIG. 8 is a flow chart representation of an option of the presentinvention for determining the look up table according to the thirdaspect of the present invention.

FIGS. 9A and 9B are diagrammatic representations of a mechanical brakingsystem according to a fourth aspect of the present invention, showingthe mechanical brake in an off configuration and an on configurationrespectively.

FIG. 9C is a diagrammatic representation of the brake system to aidunderstanding of the brake theory.

FIG. 9D is a plan view representation of a mechanical braking systemcomprising a primary and secondary braking system.

FIGS. 9E-9H are diagrammatic representations of a mechanical brakingsystem comprising a primary and secondary braking system.

FIG. 9I is a diagrammatic representation of an actuator rod coupled toan extending feature.

FIG. 9J is an illustrative graphical plot to show the non-linearbehaviour between the actuator force with the actuator rod stroke.

FIG. 10 is a flow chart representation of a method of brake performancemonitoring according to an option of a fifth aspect of the presentinvention.

DETAILED DESCRIPTION

The present invention has, as an object, the provision of a constructionof a braking system for an actuator comprising an electromagnetic motoras well as methods for the actuation thereof by a control system. Whilstthe below description is given in reference to a vertically orientatedtest apparatus, the present invention is not so limited and may beincorporated to another system requiring the effects offered thereby.

The control system and braking system may be incorporated into aconventional test apparatus comprising an actuator comprising anelectromagnetic motor comprising an output shaft moveably positionedwithin a coil assembly. Alternatively, the electromagnetic motor maycomprise a coil assembly mounted to an output shaft, the output shaftpositioned within a housing comprising magnetic material (known as amoving coil motor). The coil assembly comprising a coil assembly circuitarranged such that the motor is a multi-phase motor. In the option ofthe present invention described herein, the motor is a three-phase motorconfigured to produce linear and/or rotary movement of the output shaft.The actuator comprising the motor comprises one or more displacementsensors for determining a displacement of the output shaft within thecoil assembly and an encoder for calculating the displacement of theoutput shaft as a function of time. Examples of displacement sensorsinclude but are not limited to LVDT (linear variable differentialtransformer); strain gauges and rotary potentiometers. The encoderhaving a sampling frequency, the sampling frequency may be fixed orcontrollable such that the sampling frequency is between 1 kHz and 20kHz. Alternatively, displacement of the output shaft may be determinedthrough measurement of the velocity and/or acceleration of the outputshaft and subsequent integration to find displacement. An example of adevice for measuring the velocity is a velocity transducer. An exampleof a device for measuring acceleration is an accelerometer. Thesedevices can be included in the actuator using any known method e.g.incorporation into the output shaft. Further alternatively, displacementof the output shaft may be determined through differentiation of ameasurement of the absement of the output shaft and subsequentdifferentiations to find velocity and/or acceleration.

The object of the first aspect of the present invention is to produce acontrol system capable of differentiating between different movements ofthe output shaft such that the control system arrests the movement ofthe output shaft when a failure event occurs, a failure event being anuncontrolled (unauthorised, uninstructed etc.) movement of the outputshaft caused by a failure of a component of the linear motor.

As discussed in the introduction, there exist several events whichresult in a conventional control system arresting the movement of theoutput shaft. A conventional control system is configured to determinewhen a velocity of the output shaft (calculated by a motion detector 11)exceeds a predetermined threshold using the displacement measured by adisplacement sensor as a function of time. To achieve this, aconventional SSM 10 as shown in FIG. 1 comprises at least one comparator20 adapted to compare the velocity of the output shaft with apredetermined velocity threshold, for instance 10 mm/s for linearmovement and/or 30 deg/s for rotary movement. The velocity signal may bedetermined through differentiation of a displacement signal, integrationof an acceleration signal, or by measurement using a velocity detector.When said predetermined velocity threshold is exceeded, an output of thecomparator triggers a Safe Torque Off (STO) device 40 which activates abraking device. Additionally, the conventional SSM may also compriseanother at least one comparator adapted to compare the acceleration ofthe output shaft with a predetermined acceleration threshold (forinstance 30 mm/s for linear movement and/or 90 deg/s for rotarymovement) and trigger the STO device, in addition to or as analternative to the velocity comparator.

The predetermined velocity and/or acceleration thresholds may beexceeded by several events. These events include but are not limited tothose discussed in the introduction and repeated below:

1. An impulse is experienced by the output shaft resulting in amomentary large acceleration and velocity when the mechanical brake isdisengaged.

2. Working in the environment with the test apparatus, the apparatus canbe knocked accidentally, resulting in large acceleration and velocity.

3. Adjusting the load string with tools can result in impulses thatproduce large acceleration and velocity.

4. Operating the specimen holding grips can result in impulses thatproduce large acceleration and velocity.

5. When operating in a set-up mode, in which the current applied to themotor is strictly limited, ‘motor cogging’ can result in the velocityand acceleration thresholds being exceeded.

6. The uncontrolled movement of the output shaft as a result of afailure of a component of the test apparatus or any other event whichposes a risk to an operator said risks include but are not limited to:entanglement, friction or abrasion, cutting, shearing, stabbing orpuncture, impacting, crushing, or drawing-in. Wherein an uncontrolledmovement beyond a predetermined movement threshold corresponds to a riskto the operator. The predetermined movement threshold for linearmovement is optionally in the range 0 mm to 200 mm, or 0 mm to 60 mm. Inan option of the present invention, the threshold is 2 mm. For rotarymovement the predetermined movement threshold is optionally in the range0 degrees to 30 degrees.

Events 1-5 are known as false-failure events, not caused by componentfailure and/or without presenting a risk to operators, as opposed toevent 6 which is a true failure event.

The applicant identified that there is a need to differentiate betweenthe different events when the test apparatus is operating such that thefrequency of occurrences when the STO device is triggered is reducedi.e. only triggered by a (true) failure event and not by a false-failureevent. Therefore, leading to a lower probability of an operatorswitching off the control system when dissatisfied by the high frequencyof occurrences (false-failures).

In the present invention, the applicant noted that if a signalrepresenting the velocity or acceleration of the output shaft is brokendown into discrete periods of time, analysing the frequency spectrum ofeach period can be used to identify and differentiate between the eventswhich cannot be differentiated by using conventional comparator methodsinvolving examining only the velocity and/or acceleration in timesignal. Fourier transform is one example in the present invention toprovide a frequency domain representation of the signal representingeither the velocity or acceleration of the output shaft.

As such, according to a first aspect of the present invention, a controlsystem for controlling a braking system is provided. The control systemand braking system may be incorporated into test apparatus comprisingactuator comprising an electromagnetic motor comprising an output shaftmoveably positioned within a coil assembly. A moving coil motor is notexcluded from the scope of the invention. The braking system comprises ameans for generating a signal based on the movement of the output shaft.For the purpose of explanation of the present invention, such a means isgenerally known as a linear and/or rotary motion detector 111. In anoption of the present invention, the motion detector comprises one ormore displacement sensors for determining the displacement of the outputshaft within the coil assembly and an encoder for producing thedisplacement of the output shaft as a function of time. The encoderhaving a sampling frequency, the sampling frequency may be fixed orcontrollable such that the sampling frequency is between 1 kHz and 20kHz. In an option of the first aspect of the present invention theencoder has a sampling frequency of 10 kHz. In an option of the presentinvention, the velocity of the output shaft can be determined using avelocity transducer. In another option of the present invention, themotion detector comprises an acceleration detector, such as anaccelerometer. In another option of the present invention, the linearand/or rotary motion detector comprises at least one of: a displacementdetector, a velocity detector or an acceleration detector.

The control system, according to the first aspect of the presentinvention, comprises a safe speed monitor (SSM) 110 configured toidentify an uncontrolled movement of the output shaft using thefrequency spectrum of the velocity and/or acceleration signal. The SSMis configured to receive an input signal from the motion detector 111chosen from at least one of the displacement detector, the velocitydetector or the acceleration detector whose signals correspond to themotion of the output shaft. The SSM may additionally be configured todetermine a velocity signal and/or an acceleration signal correspondingto the velocity and/or acceleration of the output shaft based on theinput signal being any one of: the displacement, the velocity or theacceleration of the output shaft.

FIG. 2 represents an option of the present invention wherein the motiondetector comprises a displacement detector, the SSM is configured todetermine the velocity signal and/or acceleration signal correspondingto the motion of the output shaft based on the displacement bydifferentiating the signal with respect to time (d/dt).

The control system as shown in FIG. 2 comprises a first channel 120comprising a first filter 121 for attenuating one or more frequencycomponents of the velocity signal. In an option of the first aspect ofthe present invention, the first filter is a finite impulse response(FIR) filter. Whilst the present invention does not wish to exclude theprovision of the first filter being a software filter, a delay in theprocessing of the signal between receiving the output of thedisplacement detector and applying the first filter as a software filterusing components available at prices suitable for inclusion in acommercially saleable product is not short enough to satisfactorilylimit the movement of the output shaft. As discussed above, a softwarefilter would make use of a mathematical operation whereby a frequencydomain representation of the velocity signal is calculated, e.g. Fouriertransform, specific frequencies would then be attenuated. The presentinvention therefore acknowledges that some software filters may besuitable for the present purpose but are not practicable in relation tocommercial requirements. Furthermore, within software filters there aremore opportunities for errors or faults to be present leading to areduction in safety and an increase in production and maintenance costs.Therefore, in an option of the present invention, the first filter isimplemented in hardware as an analogue or a digital filter.

As shown in FIG. 2, an option of the first aspect of the presentinvention can include a second channel 130 comprising a second filter131 for modifying the acceleration signal by attenuating one or morefrequency components of the acceleration signal in the frequency domain.In a further option, the second filter may be a finite impulse response(FIR) filter. As with the first filter, the second filter may beimplemented in software or in hardware. The reader should be aware thatthe first aspect of the present invention, whilst described with boththe first and second channels, could be implemented with only one of thefirst or second channels, i.e. only the velocity signal or theacceleration signal is modified by a filter.

The following description of the first or second filters uses a FIRfilter as an example, however any other form of filter that produces thedesired signal attenuation may be implemented. A typical FIR filtercomprises a delay line having N stages, each stage having apredetermined coefficient. The selection of the number of stages andtheir respective coefficients is dependent on the desired functionalrequirement of the filter. The choice of coefficients for each stagewill affect the function or ‘shape’ of the filter, i.e. whether it is alow-pass filter, a band-filter or high-pass filter. Increasing thenumber of stages will increase the delay between receiving the output ofthe encoder and outputting a signal from the filter; however, it willincrease the precision of discrimination of the filter.

In analysing the frequency spectrum of each occurrence that causes afailure event or false-failure event, it can be noted that the frequencyspectrum of failure events is substantially lower than that of thefalse-failure event which occur momentarily. By designing an ideallow-pass filter, equating to the characteristic frequency spectrum of afailure event, it is therefore possible to filter out the false-failureevents such that these events do not result in the triggering of the STOdevice. To determine the parameters of the ideal low-pass filter, anaverage characteristic frequency spectrum of several failure events canbe determined computationally or by hand-calculation. Once determined,the ideal low-pass-filter is chosen such that the filter will remove allfalse-failure events—by attenuating frequency components in the velocityand/or acceleration signals present in the frequency spectrum due tofalse-failure events—but still capable of detecting outlier failureevents. An outlier failure event being one with frequency spectrum thatis statistically unlikely. An example of the frequency response of anideal filter for the first filter is shown in FIG. 3A and an example ofthe frequency response of an ideal filter for the second filter is shownin FIG. 3B.

A compromise must be made between the precision of the discrimination ofthe filter and the delay introduced as the number of stages of thefilter increases. In an option of the present invention, the secondlow-pass filter is chosen to have 20 stages and therefore 21coefficients. In a further option, the first low-pass filter is chosento have 60 stages and therefore 61 coefficients. The number of stageswithin the second filter is chosen to be less than the number of stagesof the first filter to reduce the reaction time of the control system.The sooner a failure event is detected the less momentum is built up andthe less risk presented to the operator. In an alternative option of thefirst aspect of the present invention, each filter may have any numberof stages and coefficients or alternatively the second filter may havemore stages than the first filter.

The result of the application of the first and/or second filter, to thefiltered velocity or acceleration signal respectively, is that only truefailure events can result in the arresting of the output shaft bytriggering a STO device 140. The filtered velocity signal, having beeneither passed without attenuation or with attenuation by the filter, istransmitted to a first comparator 122 on the first channel configuredcompare the filtered velocity signal with a predetermined velocitythreshold. This is implemented similarly by a second comparator 132 onthe second channel with the filtered acceleration signal and apredetermined acceleration threshold. An option of the first aspect ofthe present invention has the predetermined velocity threshold at about10 mm/s for linear movement and at about 30 deg/s (for example 29.8deg/s) for rotary movement and the predetermined acceleration thresholdat about 30 mm/s² for linear movement and at about 90 deg/s² (forexample 89.4 deg/s²) for rotary movement. Any other values may be chosenand are not outside the scope of the present description, other valuesare chosen in order to change the sensitivity of the SSM; for instancethe predetermined velocity threshold is optionally in the range 0 mm/sto 100 mm/s for linear movement and/or 0 deg/s to 360 deg/s for rotarymovement; and/or the predetermined acceleration threshold is optionallyin the range 0 mm/s² to 500 mm/s² for linear movement and/or 0 deg/s² to1000 deg/s² for rotary movement.

The result of the identification by the first and/or second comparatorsof the filtered signals is that the STO device is only triggered if afailure event occurs.

Expressed alternatively with reference to the flow diagram of FIG. 4 andthe graphs of FIGS. 5A & 5B: a raw velocity signal 125 corresponding tothe velocity of the output shaft is input to a filter, the filter isconfigured to attenuate frequency components of the velocity signal thatare due to false-failure events. Therefore, the filter is tailored suchthat frequency components within a characteristic frequency spectrum,corresponding to failure events, are not attenuated and those not withinthe spectrum are attenuated; this profile is known as a predeterminedfrequency profile. The filtered signal 126 is then input into acomparator, comparing the amplitude of the velocity signal with apredetermined velocity threshold 127. If the predetermined velocitythreshold is exceeded, the STO device is triggered and the output shaftis arrested. The same method can be applied equally to an accelerationsignal corresponding to the acceleration of the output shaft as shown inFIGS. 5C & 5D, wherein references 135, 136 and 137 correspond to the rawacceleration signal, filtered acceleration signal and the predeterminedacceleration threshold respectively. As can be seen from FIGS. 5A and 5Cthe raw velocity and acceleration signals—in particular peaks 128 and138—would result in the trigger of the STO device for a traditional SSM,however, following filtering to remove false-failure events from the rawsignals, the STO is not triggered when the comparison with the requisitethresholds is performed using the filtered signals i.e. the peaks inacceleration and velocity no longer exceed the predetermined thresholds.

The first and/or second channel may each comprise a decimator. FIG. 2shows an option of the first aspect of the present invention in whichthe first channel comprises a decimator 123, converting the frequency ofthe input velocity signal from the sampling frequency of 10 kHz to about200 Hz. It should be understood that the decimator may reduce the inputsignal frequency to any chosen frequency. Decimation is performed inorder that a filter is sufficiently discriminatory; to ensure this thesample frequency of the input velocity or acceleration signals should beclose to a corner frequency of the filter.

In a further option of the first aspect of the present invention,signals from the SSM are fed into a watchdog timer 150. The watchdogtimer can be configured to determine component failure within the SSM.During normal operation, the watchdog timer monitors the signals withinthe SSM, should the watchdog detect a fault, e.g. if the signal fails topropagate through the SSM to the watchdog timer, the watchdog timergenerates a timeout signal which triggers actuation of the STO. Thewatchdog timer can be configured to receive inputs corresponding to theinput or outputs of the components within the SSM and/or from the motiondetectors. In a chosen option of the present invention shown in FIG. 2,the watchdog timer is configured to receive an input from the SSM, afterthe velocity and/or acceleration signals have been filtered as theyenter the comparator such that performance of the filters can bemonitored. In an option of the present invention, the input to thewatchdog timer is the filter velocity and/or acceleration signal, if thewatchdog does not receive the input within a predetermined time-periodsince the last input was received, the STO is triggered.

In an option of the first aspect of the present invention, a second SSM170, identical to the first SSM, shares and is communicatively coupledto the STO device and the displacement detector to provide redundancy,via a cross check, in case of component failure in either the first orsecond SSM and/or to ensure accurate actuation of the STO device. Thefirst and second SSM independently determine if the input signalcomprises a failure event.

In an option of the present invention, if the outputs of the first andsecond SSM do not equate, within the cross check, that a failure eventhas occurred, the STO will not trigger the braking systems.Alternatively, the STO will trigger the braking systems regardless ofwhether the SSM′ outputs equate, an error is then flagged so that theoperator can be made aware of a possible fault in one of the SSMs.

In another option of the first aspect of the present invention, the SSMcomprises an error manager 160, the error manager configured tooptionally receive inputs from at least one of the first channel, thesecond channel and the motion detector(s). The error manager generates asignal for delivery to a user, identifying which component or signal ofthe control system is responsible for triggering the STO device. Forinstance, if the filtered acceleration signal exceeds the accelerationthreshold, the error manager will generate a signal representing thatthe acceleration threshold has been exceeded. In the instance where oneor more components of the control system trigger the STO device, theerror manager can generate a signal representing such an event. In anoption of the present invention comprising the cross check and the firstand second SSM, the SSMs may share an error manager, or each maycomprise a separate error manager.

While the object of the first aspect of the present invention is toproduce a control system that increases the accuracy with which failureevents are identified, the second aspect of the present invention hasthe object of increasing the speed with which, once an event isidentified as a failure event, the output shaft is arrested.

As discussed in the introduction and shown in FIG. 6A, conventionally anSTO device comprising a mechanical relay 210 is used to connect each ofa plurality separate coil loops of the coil assembly to each other suchthat the current induced in the shaft generates a Lorentz forcesufficient to arrest the shaft. The mechanical relay comprises anelectromagnetic coil, controlled by the SSM, used to open and close anelectrical circuit 211 for which the coil assembly is connected to. Useof the mechanical relay is chosen for its extended lifetime andreliability in function throughout. However, current mechanical devicesare unable to connect the separate coil loops of the coil assembly insufficient time to comply with stopping the output shaft before themaximum distance that the shaft can travel, e.g. 200 mm for linearmovement and 360 degrees for rotary movement.

The voltage used to drive the electric motor are necessarily high inorder that the magnetic field generated in the coils produces asufficient Lorentz force on the output shaft. A mechanical switch istraditionally used to remove the isolation between the separate coilloops such that this high voltage is not able to damage low voltagecomponents of the STO and SSM.

To reduce the delay between decision to arrest and the output shaftcoming to rest, the second aspect of the present invention comprises anSTO device having a solid-state relay (SSR) circuit switch 220.

The solid-state relay circuit switch optionally comprises a back-to-backMOSFET 224 design as shown in FIG. 6B; FIG. 6B represents an STO devicefor a coil assembly comprising two separate coil loops. Under normaloperation, i.e. when the STO device is not triggered, each of theplurality of separate coil loops is isolated. An SSR circuit ispositioned between the each of the plurality of separate coil loops,such that when the STO device receives an instruction input from the SSMto arrest motion of the output shaft, the SSR circuit is switched froman ‘off’ configuration to an ‘on’ configuration, and current can flowthrough the right-hand side of the SSR circuit therefore removing theisolation between the loops.

In an option of the second aspect of the present invention, the problemof high voltages within the coil assembly is overcome through theinclusion of an opto-isolator 221. The opto-isolator enables the SSRcircuit to be switched without exposing the low voltage circuit to highvoltages from the coil assembly by isolating the circuits. Theopto-isolator comprises a light emitting diode LED 222 and aphotovoltaic cell 223. On instruction to arrest the output shaft, asupply current activates the LED, emitted light is detected by thephotocell, and within the back to back MOSFET circuit the SSR circuit iscompleted as current can flow, therefore connecting the separate coilloops and providing a braking effect. The supply voltage, the LED,photovoltaic cell and parameters of the MOSFET transistors are chosensuch that the switching time of the SSR circuit is minimised.

In an effort to further minimise the risk of component damage due toexposure to high voltage, the SSR circuit is connected to the coilassembly via a protection means 225. These protection means can comprisefuses, or in an option of the present invention, a Transient VoltageSuppressor (TVS) Diode such that in the case of high voltage (beyondthose expected in normal operation) the MOSFET transistors areprotected.

For a three-phase motor, two solid state relay circuits are utilised, asdepicted in FIG. 6C. A first SSR circuit 226 is used to connect a firstand a second coil loops; a second SSR circuit 227 is then used toconnect the second coil loop to a third coil loop. To provideredundancy, in an option of the present invention, the STO devicecomprises a set of redundant SSR circuits configured for connecting thecoil loops of the coil assembly. The redundant SSR circuits aretriggered by identical means to the primary SSR circuits. For instance,in an option of the present invention wherein the motor is a three-phaselinear motor, inclusive of the redundancy circuits, there are four SSRcircuits within the STO device.

Although the present invention according to the first aspect enables thecontrol system to sufficiently differentiate between failure events andfalse-failure events, it is preferable to reduce or eliminate the causesthereof.

The object of the third aspect of the present invention relates to theimpulse experienced by the output shaft when the mechanical brake isreleased from or applied to the output shaft, i.e. during the initialstart-up of the testing apparatus. The impulse or “jolt” experienced isas a result of the mechanical brake disengaging with the output shaft.However, this impulse or jolt can be considered as a failure event asdiscussed with respect to the first aspect of the present invention andcan trigger a fault.

According to an option of the third aspect, the present inventionprovides a means to reduce movement of the output shaft caused by themotion of the output shaft when the output shaft is released by themechanical brake, e.g. when the testing apparatus is switched on andheld by the force induced by the coil assembly. The current required tomaintain the position of the output shaft within the coil assemblyvaries with the position of magnets within the output shaft in relationto the three-phases of the coil assembly. Thus, it is possible todetermine for the test apparatus a current vs. position look up table,as shown graphically in FIG. 7 where the position of the output shaftalong the length of motion is plotted against necessary current. Theplot shown in FIG. 7 shows the variation of the current applied to thecoil assembly versus the position of the output shaft for a 3-phasemotor design. In FIG. 7, there are two plots, a sinusoidal plot whichrepresents the current in the motor and a straight line plot at aconstant current that represents an average of the current in the firstplot. The look-up table is derived from the sinusoidal plot andrepresents the current to be applied to the coil assembly necessary tohold or resist movement of the output shaft. For example, at every 2 mmstroke of the output shaft, the corresponding current is noted in alook-up table or database. This is repeated for the full stroke of theoutput shaft. The straight line provides a base line for which thecurrent is either added to or subtracted from, for the purpose offeeding into the electronics of the testing machine.

According to the third aspect of the present invention, a controllerdetermines the position of the output shaft and correlates the positionof the output shaft to a current in the look-up table necessary to holdthe output shaft at that given position such that when the current isapplied to the coil assembly, the output shaft is resisted frommovement.

The movement control system for a motor comprises a mechanical brake andthe controller configured to receive a position of the output shaftwithin the coil assembly and to correlate the position with a currentrequired to apply to the coil assembly such that motion of the outputshaft is resisted when a mechanical brake is released. The controllercan correlate the position with current by calculation or by referencinga look up table of current vs position. The look up table can becalibrated during manufacture or alternatively may be calibrated eachtime a specimen is loaded for testing as the mass of the specimen and achosen load cell will change the values of current at each position. Forexample, the testing machine can be run such that the output shaft isrun through a range of different stroke positions and the current in thecoil assembly is measured at the different stroke positions to generatea plot of current in the coil assembly versus stroke position of theoutput shaft as shown in FIG. 7. Typically, the current is noted at 2 mmincremental positions of the output shaft and represents the acceptableleeway in the movement of the output shaft when the mechanical brake isdisengaged. However, other incremental positions of the output shaft ispermissible in the present invention

Calibrating the look-up table can be performed using the followingmethod, also shown in FIG. 8:

Step A1—determining a first current needed to hold the output shaft in afirst position without engaging the mechanical brake.

Step A2—determining a second current needed to hold the output shaft ina second position without engaging the mechanical brake.

Step A3—optionally—determining a third current needed to hold the outputshaft in the second position without engaging the mechanical brake

Step A4—optionally—repeating step A3 for successive positions of theoutput shaft.

Step A5—storing the first position and first current, and others in thelook-up table.

In an option of the present invention, the look-up table is calibratedfor every position along the entire length of travel of the outputshaft.

The above calibration process can be performed automatically, e.g.during start-up of the testing machine, whereby each time the mechanicalbrake is released at a given position along the coil assembly afeed-back loop is used to vary the current in the coil assembly untilthe output shaft is held at that position. Movement of the output shaftcan be detected by a motion sensor. The position of the output shaft ismeasured in a conventional way and comprises an optical absolute encoder(transducer) fixed to the motor body or any fixed part of the testingmachine cooperating with a graduated scale attached to the moving outputshaft. As the output shaft moves, the graduated scale moves relative tothe optical absolute encoders. A determination of the position of theoutput shaft is thus measured from the graduated scale. This is repeatedat different positions of the output shaft along the coil assembly. Thecurrent values to hold the output shaft at the different position withinthe coil assembly are stored in a look-up table.

Through the implementation of the look-up table, there is minimal or nomotion of the output shaft each time the mechanical brake is disengaged,in an option of the present invention motion is reduced to less than 2mm.

Industry standards require shoes of the mechanical brake to be ratedsuch that they reliably engage throughout their life. The mechanicalbrake can comprise a tubular brake shoe that is located concentricallyround the output shaft or brake shaft of the motor. The brake shoe‘inside diameter’ is slightly larger than the output shaft or the brakeshaft of the motor. Conventional test apparatus monitor the number oftimes that the mechanical brake has been actuated for each brake shoeand recommend to a user that the shoe be replaced once a predeterminedactuation threshold has been exceeded.

The purpose of the mechanical brake is to prevent the output shaftdropping under gravity when the linear motor is turned off. Without amechanical brake, a risk hazard to an operator's hand or fingers ispresent when accessing the test workspace. The source of the crushingforce is derived from the falling mass of the actuator shaft.

The object of the fourth aspect of the present invention is to provide amechanical brake that enables improved engagement reaction timing overthat achieved from conventional linear motor brakes.

A brake 400, mounted on the frame of a test apparatus incorporating theoutput shaft 401 of a linear motor as shown in FIG. 9A, 9B, comprises anelongate plate 410 provided with a space for receiving the output shaft,wherein the space in the elongate plate according the present inventionmay include, but is not limited to, a hole, an opening, a cut-out, anaperture or a slot (made by any suitable means). For the purposes ofdescription, in the following options the space in the elongate plate isa hole 411. The elongate plate 410 plane may be positioned such that itis perpendicular to the output shaft axis 403 or direction of movementof the output shaft 401 when the brake 400 is not engaged. The shape andsize of the hole is just larger than the cross-sectional shape and sizeof the output shaft (e.g. to provide radial clearance) so as tofrictionally engage with the output shaft and/or the brake shaft whenthe elongate plate is tilted.

Alternatively, the shape and size of the hole may correspond to theoutput shaft such that the edges of the hole are proximate to but not incontact with the output shaft and/or the brake shaft when the brake isnot engaged. The tilt angle with respect to an axis perpendicular of theoutput shaft axis 403 can be approximately 0.1-15 degrees, or morepreferably approximately 1-10 degrees, further preferably approximately2-4 degrees. The tilt angle of the elongated plate is controlled bycontrolling the geometry or size of the spacing. For example, a largespacing or hole would cause the elongated plate to tilt further as theedges of the spacing would need to travel further before coming intocontact with the output shaft. Thus, the further away the edges of thehole are from the output shaft, the greater the tilt angle of theelongated plate necessary for the edges of the hole to be in contactwith the output shaft when engaged. To help the elongate platefrictionally engage with the output shaft, more particularly the hole ofthe elongate plate, the output shaft comprises a brake shaft 402 towhich the brake engages. In an option of the present invention, thebrake shaft and the output shaft can be integrally formed as a singlebody; alternatively brake shaft 402 is formed as a sleeve around theoutput shaft or is an extension of the output shaft. The brake shaft canbe fastened to the output shaft by any suitable means, e.g. adhesive ormechanical fasteners. The elongate plate is pivotally, or hingedly,mounted or coupled at one end 412 and is contacted at its other end 413by a brake inhibitor 420 which is arranged to hold the plate in aposition such that there is sufficient clearance between the edge of thehole and the brake shaft and/or the output shaft to allow the brakeshaft and/or the output shaft to pass freely through the hole withoutcontacting the edge of the hole. Preferably, this is when the plate isat right angles to the axis or direction of movement of the brake shaftand/or the output shaft.

The brake inhibitor is an electrically actuated device 420 such as asolenoid 423, whereby the electrically actuated device can be anelectrically operated holding device and/or electrically operatedlifting device. In this case, a linear solenoid, which in normal usewhen electrical power is available, holds the plate in its “off”condition by means of an actuator rod 421, but when electrical power isnot available exerts little or no force on the elongate plate and thusallows it to pivot under the action of gravity so that the edge of thehole contacts the brake shaft and/or the output shaft and causes thebrake shaft to frictionally engage and jam with the edge of the hole asit tries to drop, also under the action of gravity. Alternatively, theelongate plate is biased to jam against the brake shaft, i.e. in the“on” condition, and the actuator rod is arranged to disengage theelongate plate, more particularly the hole of the elongate plate fromthe brake shaft in the “off” condition. In the “on” condition, twocontact points (preferably diametrically opposed but offset vertically)are made to engage with the output shaft. This is the “brake on” or“engaged” state and occurs simultaneously with the loss of power to themotor. Therefore, at the point of power loss to the motor, the brakeshoe and/or the elongate plate are “dragged” downwards. This allows thebraking action to be self-energising which means that the mass of theoutput shaft acting downwards under gravity (weight) generates thebraking force needed to hold the output shaft.

Movement of the elongated plate is by freely pivotally coupling theelongated plate, more particularly an end of the elongate plate to theactuator rod so that vertical movement of the actuator rod causesmovement of the elongate plate. In the particular embodiment of thepresent invention, the actuator rod comprises a retaining groove 422into which the plate sits. Other coupling means are included in thescope of the present invention, for example, a ball and socket joint. Avertical solenoid offers advantages over the rotary solenoids used inconventional brake systems. In rotary solenoids, the brake inhibitorrotates downward when there is no electrical power available, thereforethe action of gravity is transmitted through the pivot point whichintroduces resistance. For linear solenoids, the actuator rod dropsunder gravity without resistance from a pivot point or otherwise. Itsacceleration is therefore closer to that of gravity, resulting inearlier engagement of the brake with the brake shaft.

It is important that in the “off” condition the brake imparts nofriction to the brake shaft as shown in FIG. 9A. This is achieved in thepresent invention by means of a limit stop 430 fixed to the frame andagainst which the brake inhibitor presses the plate. To increase thesensitivity of the mechanical brake to frictionally engage with thebrake shaft, it is necessary that the hole of the elongate plate isaccurately positioned in close proximity with the brake shaft. Accuratepositioning of the hole with respect to the brake shaft is achievedusing an insert 440 which fits in the hole of the elongate plate. Theinsert comprises a space, wherein the space according the presentinvention may include, but is not limited to a bore, an opening, acut-out, an aperture, a slot (made by any suitable means) through whichthe brake shaft and/or output shaft passes. For descriptive purposes,the space of the insert is herein described as a bore 441. When thebrake is in the “on” condition, the edge of the bore binds or jamsagainst the brake shaft, thereby preventing further movement of thebrake shaft. To provide the necessary frictional force between the edgeof the bore and the output shaft, optionally the insert comprises abrake shoe that binds against the outside surface of the output shaftwhen the brake is in an “on” condition.

A fundamental feature of the mechanical brake of the present inventionis that the braking action is self-energising. This means that the massof the output shaft acting downwards under gravity generates the brakingforce needed to hold the output shaft. This braking force is directlyproportional to the weight or any further applied load to the outputshaft. The ratio of brake force to the output shaft is determined by thegeometry of the elongated plate and/or an insert. The brake forces actnormal to the output/brake shaft and create a frictional holding force.Provided that the friction coefficient is above a critical value thebrake will self-lock. The forces necessary to hold the output shaft whenthe brake is in the “on” condition can be explained by considering themoment of the elongate plate when the bore binds against the brakeshaft. This is expressed mathematically by the equation below withreference to FIG. 9C:

Fd=Rt cos θ  (1)

where:—

F=force of mass to be supported, e.g. the weight of the output shaft;

R=reaction force of the brake plate acting on the brake shaft or outputshaft;

t=is the thickness of the brake plate;

θ=is the angle that the edge of the bore makes with the output shaftwhen the brake is in the “on” condition;

d=distance between the pivot point (fulcrum) and the centre of the bore441;

μ=coefficient of friction between the edge of the bore 441 in contactwith the brake shaft and the brake shaft.

For small angles of pivot, cos θ can be approximated to 1, i.e. Fd=Rt.

When the brake is in the “on” condition, the free end of the elongatedplate 412 pivots about a pivot point or fulcrum 450. The torque, T, ormoment, M, acting on the elongate plate at the point when the bore bindsor jams against the brake shaft is given by:—

T=Fd  (2)

The braking forces acting at two points of contact with the brake shaftwhen the bore binds or jams against the brake shaft as a result offriction is given by:—

Braking force=2μR  (3)

Therefore, the braking force necessary to arrest the brake shaft whenthe bore binds against the brake shaft must be equal to or greater thanthe force due to the mass of the output shaft, i.e. 2 μR≥F.

A critical friction, μ_c is reached when the braking force is equal tothe weight of the output shaft, 2μ_(c)R=F.

In the particular embodiment of the present invention as shown in FIG.9B, the elongate plate tilts about a fulcrum positioned on a resilientmember 450. The fulcrum is positioned such that the elongated platepivots about a point which is offset from the axis of the output shaft.In the “off” condition, the limit stop prevents titling of the insertand thereby, ensures that there is clearance between the edge of thebore of the insert and the brake so permitting the brake shaft and/orthe output shaft to travel through the bore and/or the hole. In theparticular embodiment of the present invention, the insert comprises aprojecting portion having an external surface that cooperates with thelimit stop. In one example of the present invention, the externalsurface of the projecting portion has a frusto-conical shape that isarranged to be received in a complementary profile of the limit stop.

The insert is an optional addition, the same function could be achievedby shaping a portion of an upper surface of the plate to a profile thatcooperates with the profile of the limit stop. In the present invention,the insert comprises a material suitable for providing a braking effecton the brake shaft such as rubber (e.g. brake shoe). In an option of thepresent invention, the insert comprises phosphor bronze. In anotheroption, the insert comprises a ceramic. In an option of the presentinvention, the plate comprises aluminium or another suitable stiffmaterial. In an option of the present invention, the plate comprises aceramic. When the elongate plate comprises ceramic, the insert may alsobe ceramic or another braking material and the plate and insert can beintegrally formed as a single body.

The force acting on the actuator rod is non-linear at each stroke of theactuator rod. This can be seen in the plot of force acting on theactuator rod against the length of stroke of the actuator rod shown inFIG. 9J and is influenced by the degree of overlap of the actuator rodwith the coils of the electrically actuated device. The force decreasesas the actuator rod moves away from a retracted position into anextended position. This can be problematic as the holding force at thislevel of extension may not generate sufficient holding force to hold theelongated plate and/or output shaft in the “off” condition. Thus, at thepoint of the brake release (“off” condition), the actuator rod will beat a large stroke position where it produces the lowest force.

As the actuator rod has a rigid connection to the elongated plate, itmay be possible for the actuator rod to be in a retracted position, i.e.hitting the end stop of the solenoid, in the “off” condition before theelongated plate is seated up against the limit stop 430. In this case,the mechanical brake may still be at least partially engaged with theoutput shaft causing the mechanical brake, more specifically the insert(brake shoe), to “rub” against the output shaft.

Equally, it may be possible for the elongated plate to be seated upagainst the limit stop 430 when the actuator rod is in a partlyretracted position. According to FIG. 9J, at the partially retractedstate of the actuator rod, the solenoid produces an increasingly lowerholding force.

In order to overcome these “drawbacks”, the coupling between theactuator rod and the elongated plate comprises an extendable feature.The extendable feature 501 (coupling device) comprises a pre-loadedresilient member 502 (e.g. a pre-loaded tension spring or a resilientlyextendible member) attached to an end of the actuator rod coupled to theelongated plate (or lifting end) such that in the relaxed state of theresilient member, the expandable member expands causing the actuator rodto retract further and thereby, causing the elongated plate to be seatedagainst the limit stop in the “off” braking condition, i.e. butt upagainst the limit stop 430. At this orientation of the elongated plate,the clearance between the bore of the elongated plate and the outputshaft is sufficient to allow the output shaft to move without rubbingagainst the elongated plate. The expandable coupling allows the lengthof the actuator rod to be expandable by approximately 3.5 mm. However,it is feasible that for different levels of loading on the resilientmember, the extension can have a range between 0.1 mm to 10 mm. Theexpendable feature provides some additional give to the coupling betweenthe actuator rod and the elongated plated to allow the elongated plateto be seated up against the limit stop 430 when retracted.

In the particular embodiment of the present invention as shown in FIG.9I, the extendable feature comprises an insert attached to the liftingend of the actuator rod and is moveable within a grooved or flangedhousing 503. The insert is held within the flanged housing by apre-loaded resilient member. The groove or flange of the housingcooperates with the elongated plate so that movement of the actuator rodcauses movement of the elongated plate. When the pre-loaded resilientmember is in a relaxed state, the insert moves upwards within theflanged housing and thereby, allowing the actuator rod to be more soretracted within the electrically actuated device or solenoid.

Even if the elongated plate is seated up against the limit stop 430, theextendable feature 501 allows the actuator rod to be more retractablewithin the electrically actuated device when the mechanical brake isreleased, i.e. in the “off” condition. At the point of brake release,when the electrically actuated device is pulsed with current, theextendable feature allows the actuator rod to be instantly extended bythe full travel of the resilient member and thus, allows greaterretraction of the actuator rod within the electrical actuated devicehousing, i.e. coils, than is achieved without the extending feature. Inan ideal scenario for maximum holding force, the actuator rod is fullyretracted with the electrically actuated device housing. As a result,the actuator rod is operating closer to the high force end of theelectrically actuated device, preferably the high force end of theelectrically actuated device as shown in FIG. 9J. Any of the actuatorrods of the electrically actuated devices may comprise an actuator rodextendable feature of the present invention.

The present invention further differs from conventional brakes such asthat of EP 2 054219 B1 (MEAD, Graham) 6 May 2009 in that the elongatedplate pivots about a fulcrum encouraged or biased by a resilient member460. The resilient member is arranged to provide a force acting torotate the plate into its “on” condition as shown in FIG. 9B, i.e. theresilient member 460 biases the elongated plate into its “on” condition.In an option of the present invention, the resilient member is a springor another suitably elastic member. The resilient member can be acompression spring, such that when the brake is in the “off” conditionthe spring is compressed by the action of the actuator rod lifting theplate. In the “on” condition, the spring decompresses, increasing therate at which the plate accelerates downward.

In an alternative option, the resilient member is an extension spring,positioned below a lower surface of the plate such that when the brakeis in the “off” condition the spring is extended by the action of theactuator rod lifting the plate. In the “on” condition, the springreturns to a non-extended state, increasing the rate at which the plateaccelerates downward. In an option of the present invention, theresilient member comprises both an extension spring positioned below alower surface of the plate and a compression spring positioned above anupper surface of the plate. Alternatively or additionally, the elongatedplate can be biased in the “on” condition by the provision of aresilient member 461 positioned on the actuator rod 421 and acting tobias the elongated plate in a tilted or pivoted orientation which inturn causes the edge of the bore to jam against the braking shaft. Forexample, in the particular embodiment of the present invention as shownin FIG. 9E, an extension spring (not shown) can be positioned along theactuator rod 421 and located between the groove 422 and the solenoidbody. As with the other resilient members, extension of the springincreases the rate at which the elongated plate accelerates downwards(i.e. increases the rate at which the brake is engaged into the “on”condition).

The inclusion of the resilient member has the additional effect suchthat when the brake is disengaged from the brake shaft, the impulseexerted by the brake on the brake shaft is reduced. The resilient membercan be included in the conventional brake designs such as that of EP 2054219 B1 (MEAD, Graham) 6 May 2009, in which rather than a linearsolenoid, a rotary solenoid is used.

When electrical power is restored, i.e. the brake is instructed torelease, a high force is required and it is preferred to inject a pulseof energy from one or more capacitors into the solenoid to release thebrake.

To move and/or hold the elongated plate from an “on” braking conditionto an “off” braking condition, it is necessary that the electricallyactuated device (brake inhibitor) 420 is able to lift the elongatedplate so as to allow the elongated plate to pivot about the fulcrum 450and thereby, allow the brake shaft and/or the output shaft to passfreely through the hole without contacting the edge of the hole.However, due to the loading of mechanical brake by the output shaft, themechanical brake, more specifically the elongated plate is initiallyjammed against the output shaft, and more energy maybe required toinitially lift the elongated plate than is able to be provided by asingle electrically actuated device when in an “on” condition. Oneoption would be to upsize the electrically actuated device to a morepowerful electrically actuated device. However, this has the drawbackthat it usually requires more power consumption dissipating more heatwith the resultant effect of requiring additional cooling. Additionaldrawbacks are that it is generally larger in size than is permissible bythe size constraints of the testing apparatus. Another option, would beto have a second electrically actuated device that works in tandem withthe aforementioned electrically actuated device (first electricallyactuated device) to lift and hold the elongated plate. However, thisstill suffers from the problem of the need to provide additional powerto operate both of the electrically actuated devices. The presentapplicant realised that the greatest lifting force is only requiredmomentarily when initially lifting the elongate plate away from the “on”condition, i.e. to provide a momentary lifting “kick” action to theelongated plate. This momentary lifting “kick” action may be to providethe necessary force to initially disengage the insert(s) (or brake shoe)from the surface of the output shaft 401 or brake shaft. Once the brakeshoes (inserts) have been disengaged from the output shaft or brakeshaft, not much energy is required to hold the elongated plate in the“off” condition. The elongated plate can, therefore, be held in the“off” condition by supplying power to the first electrically actuateddevice alone. The second electrically actuated device (e.g. anelectrically operated lifting device 424) may then be put in ade-powdered state and therefore becomes redundant until additionallifting force is required to disengage the elongated plate from the “on”condition, more particularly the bore of the elongated plate, from theoutput shaft or brake shaft.

In comparison to the first electrically actuated device whereby theactuator rod of the first electrically actuated device engages with theelongated plate in both the upward and downward direction of theactuator rod by means of the retaining groove 422, i.e. movement of theactuator rod corresponds with the movement of the elongated plate, theactuator rod of the second electrically actuated device is onlypermitted to provide a lifting action of the elongate plate. This is toprevent the weight of the elongated plate influencing the mechanicalbrake when power to the second electrically actuated device has beenremoved, i.e. the actuator rod does not push the elongated plate downwhen power to the second electrically actuated device is removed. In theparticular embodiment of the present invention as shown in FIG. 9E, theactuator rod of the second electrically actuated device comprises asupport surface that engages with the underside of the elongated platein the upward direction but disengages from the elongated plate in thedownward direction. For example, the support surface can be a plate thatbecome in contact with the underside of the elongated plate when movingin the upward direction and so providing a lifting action.

In order to protect the mechanical brake mechanism from excessive loads,the mechanical brake of the present invention further comprises a meansfor limiting the degree of braking force which can be generated by thepivoting plate arrangement to enable ready resetting of the apparatus.This is achieved by arranging a first end of the plate to pivot aboutthe fulcrum that is resiliently mounted by means of a spring 451, oranother resilient member, and a second end of the plate provided with aprojection 453 or an overload stop arranged to be spaced from a surface404 of the actuator or a mechanical brake housing positioned below theplate. The spring provides a biasing force acting in a directionopposite to the force of gravity. In this way, should excess force beapplied in a downward direction after the brake has been activated, theplate can be pulled downwards against the spring force until theprojection or overload stop contacts the surface whereupon the elongateplate will tend to pivot about the projection or overload stop againstthe spring force so as to permit movement of the brake shaft and/or theelongate shaft through the hole and/or the bore. It will be appreciatedthat should this excess force be removed, the braking action will beimmediately restored and the brake shaft held in its new position. Thespring limits the braking force to a value which can be readily releasedsimply by activating the solenoid.

The present invention, provides an improvement to the mechanical brakeof EP 2 054219 B1 (MEAD, Graham) 6 May 2009. Throughout the working lifeof the brake, the abrasion between the plate and the brake shaft resultsin the wearing down of the surface of the plate (or, if present, theinsert). As the shape of the hole in the plate changes shape and size,the brake angle at which the plate lies when the brake is in the “on”condition increases. Therefore, late in the useable life of the brake,there comes a point at which the projection contacts the surface belowthe plate even though the brake has yet to engage the brake shaft. Inthe present invention, this surface is adjustable such that the surfacecan be lowered and/or raised. Therefore, restoring the distance betweenthe projection and the surface and extending the useable life of thebrake. In another option of the present invention, the projectioncomprises an adjustable length.

Rather than the plate comprising the projection, instead, in anotheroption of the present invention, the surface 404 comprises an overloadstop 452 for contacting the lower surface of the plate. The height ofthe overload stop is adjustable. The means for adjustment may be via ascrew mechanism or any other suitable means. The stop having identicalfunction to that of the projection.

The object of the fifth aspect of the present invention is to provide amechanical brake performance monitor that enables improved monitoring ofthe mechanical brake.

The performance monitor comprises a controller configured for receivingsignals corresponding to the position of the output shaft. In an optionof the present invention, the position of the output shaft is determinedby the encoder.

During normal use, the actuation of the mechanical brake has severalphases:

1. arming—comprises removing power from the solenoid such that the platepivots and engages with the brake shaft; during the “arm” phase thebrake shaft is held in place by the electro-magnetic linear motor.

2. loaded—comprises removing power from the linear motor such thatmotion of the brake shaft under gravity is only arrested by the actionof the mechanical brake.

During the loaded phase, the brake shaft of a linear motor ‘drops’, thisis due to mechanical stresses within the brake plate. Drop is anintrinsic property of brake, a result of the materials chosen and theenergy of the system, it is not a constant however, the variance overtime is minimal. When in the braking phase, there is another effect thatmay result in motion of the brake shaft; the effect may be due toexcessive wear of the brake or a change in the frictional coefficientsof the portion of the brake plate engaging the brake shaft and the brakeshaft itself. The degree of ‘slip’—the distance through which the brakeshaft moves when in the loaded phase not attributable to ‘drop’—needs tobe monitored as increased ‘slip’ indicates a need to service or replacethe brake. Moreover, as the mechanical brake, more specifically thebrake shoe wears, the spacing between the lower surface of the elongatedplate and the overload stop 452 gradually reduces. This results in anincreasing tilt angle of the elongated plate as the elongated plate isengages with the brake shaft. If the lower surface of the elongatedplate comes into contact with the overload stop 452 in the armingcondition, then this is also an indication that the mechanical brake iscomprised or worn out. The terminology “plate”, brake plate”, “elongateplate” and “elongated plate” are used interchangeably throughout thespecification to describe the same feature.

The performance monitor is configured to determine the ‘slip’ of thebrake shaft. Owing to ‘drop’ variance being minimal but ever-present,the performance monitor determines ‘slip’ plus ‘drop’. In the presentdiscussion, the term ‘travel’ is understood to include the drop distance(i.e. travel=slip+drop). Wherein travel corresponds to the displacementof the output shaft over the duration of actuation of the mechanicalbrake. Following the method steps shown in FIG. 10, the performancemonitor monitors the travel of the output shaft:

Step B1—recording a first position corresponding to a position of theoutput shaft when the mechanical brake is initially actuated such thatthe mechanical brake is engaged with the output shaft.

Step B2—recording a second position corresponding to a position of theoutput shaft at a predetermined time after the first position isrecorded or when the output shaft comes to rest.

Step B3—determining the travel, by calculating the distance between thefirst position and the second position and/or storing the travel.

Step B4—comparing the travel with a predetermined slip threshold.

Step B5—alerting a user to the degree of travel if the travel exceedsthe predetermined travel threshold.

Step B6—shutting down the test apparatus if the travel exceeds thepredetermined travel threshold until the test apparatus has beenserviced and/or the mechanical brake examined.

A sensor, e.g. an optical switch, is used to monitor whether themechanical brake, more specifically the hole of the elongated plate, isin engaged state with the output or brake shaft or not. An “opticalflag” mounted to the elongated plate interrupts a light beam to anoptical switch each time the mechanical brake engages with the outputshaft, i.e. in a titled orientation. This generates a feedback signal toa controller indicating that the mechanical brake is in an “on”condition. When the mechanical brake is released or in an “off”condition, the elongated plate is in a horizontal condition and light tothe optical switch is not interrupted. Other sensors to monitor theposition of the elongated plated and therefore, engagement with theoutput shaft is permissible in the present invention, .e.g. rotationalsensor.

In an option of the present invention, the time between the first andsecond measurements is between 0 and 1 seconds, optionally 0.25 s.

In an option of the present invention, the predetermined travelthreshold is between 0 and 10 mm, optionally 2 mm.

In an emergency event, the output shaft undergoing uncontrolledmovement, the performance monitor records the position at which themechanical brake receives the instruction to brake as the first positionrather than position once the brake is armed.

To pre-empt the output shaft from slipping excessively, the performancemonitor may also monitor and/or record the number of actuations of themechanical brake. A firmware records/counts the number of times thesensor detects the mechanical brake engages with the output shaft. Ifthe controller determines that the number of actuations of themechanical brake exceeds a predetermined value, then a warning messageis sent to the user. For example, endurance testing shows that themechanical brake can complete in excess of 50,000 operations from theinitial setting at the building stage before the lower surface of theelongated plate contacts the overload stop.

By monitoring the number of actuations and/or the travel of themechanical brake, the performance monitor can more accurately determinewhen the mechanical brake is not fit for purpose and could cause injuryor damage. The performance monitor is configured to alert an operatorand/or third party when the mechanical brake is a risk to the operator'ssafety.

The purpose of the mechanical brake is to prevent the output shaft ofthe motor falling under gravity when the motor is turned off. Withoutsuch a mechanical brake a crush risk hazard to an operator's hand orfingers is present when accessing a test workpiece. The source of thecrushing force is derived from the falling weight of the output shaft.However, a dangerous condition could arise should the mechanical brakefail to engage with the output shaft and provide the necessary brakingaction to arrest the output shaft. For example, the brake shoe/insertcould be worn and fail to provide the necessary frictional force toarrest the output shaft. Other examples, include the electricallyactuated device fail to be in an armed position through a fault in theelectrical switching which fails to remove the power to the solenoid toallow the mechanical brake engage with the output shaft, i.e. theelongated plate is unable to move, e.g. the actuator rod of theelectrically actuated device fails to reciprocate within the solenoid.To mitigate the possibility of the mechanical brake failing, in a sixthaspect of the present invention, the mechanical brake comprises aprimary mechanical brake and a secondary mechanical brake. The secondarybrake provides a redundant (secondary) mechanical brake to the primarybraking system should the primary braking system fail. The primary brakeand the secondary brake can be based on the mechanical brake discussedin the fourth aspect of the present invention discussed with referenceto FIG. 9A and FIG. 9B, i.e. their respective elongated plates pivotabout their respective fulcrum 450, 450 a. Both primary and secondarybrakes can operate independently or dependently of one another.Optionally or additionally, both the primary and secondary brake canwork simultaneously or sequentially.

Both the primary and secondary mechanical brakes may be simultaneouslyactuated, i.e. power to their respective solenoids are turned off at thesame time. The braking action of the output shaft is shared between theprimary mechanical brake and the secondary mechanical brake. Optionally,in normal braking, both the primary and secondary brakes are engaged butonly the primary brake takes the braking load, i.e. holds the outputshaft up. In this scenario, the secondary brake is engaged but does notbecome loaded, i.e. it is in the armed condition. Since the secondarymechanical brake is in the armed condition when the primary mechanicalbrake takes up the load of the output shaft, the requirement for anadditional electrically actuated device, e.g. solenoid, the need toprovide an additional lifting action of the elongated plate of thesecondary mechanical brake to momentarily release the secondarymechanical brake is not necessary as with the primary mechanical brake.Unlike the primary mechanical brake which bears the full load of theoutput shaft, the force to lift the elongated plate of the secondarymechanical brake is not so great as it is primarily in the “armed”condition.

Focusing a greater share of the braking action on the primary mechanicalbrake rather than the secondary mechanical brake can be controlled bycontrolling the tilting action of their respective elongated plateswhich in turn is controlled by the geometric shape and/or size of thehole for receiving the output shaft of the respective elongated plates.For example, the hole in the elongated plate of the secondary mechanicalbrake is intentionally made larger and/or of a different geometric shapeto the hole in the elongated plate of the primary mechanical brake suchthat when both the primary mechanical brake and the secondary mechanicalbrake are actuated simultaneously, the primary mechanical brake engagesthe output shaft before the secondary mechanical brake and thereby,takes up the load of the output shaft.

By allowing the primary mechanical brake take up the load of the outputshaft and “arming” the secondary mechanical brake, wear of themechanical brake, specifically the brake shoe primarily occurs with theprimary mechanical brake. The brake shoe of the secondary mechanicalbrake does not experience such excessive wear as the primary mechanicalbrake since it is only loaded when the primary mechanical brake fails.In such an event and to maintain the safety of the testing device, aservice engineer changes the brake shoes of both the primary mechanicalbrake and/or the secondary mechanical brake should the primarymechanical brake fail.

The secondary brake may take the braking load (partially or wholly) inat least one of the following conditions:

a) the primary brake fails to continue to hold after any period ofsuccessful holding action; and/or

b) the primary brake fails to engage in the first instance when calledupon.

The secondary braking system can have several optional operating modes:

A. Turning output shaft ON and therefore, the mechanical brake off.Prior to “turn on”, the output shaft is supported by both of themechanical brakes (e.g. the primary and the secondary mechanical brake)since both of the mechanical brakes are in an engaged state. Turning theactuator on leads to the primary and the secondary mechanical brakesbeing released (“off” condition) in sequence or simultaneously. Thesecondary brake may be released first (no power has been supplied to themotor driving the output shaft at this point) followed by the release ofthe primary brake following a short delay between 0 and 1 second,preferably between 0.1 and 0.5 seconds, more preferably between 0.1 and0.3 seconds. Simultaneously with the release of the primary brake, poweris applied to the motor. Each brake is released by applying a high pulseof current to the electrically actuated device(s). The electricallyactuated device lifts the elongate plated to the released position bypivoting about their respective fulcrum(s).

In some cases, as shown in FIGS. 9D to 9F, the primary brake maycomprise two electrically actuated devices 421, 424 (S1 and S2) whichwork in parallel. Electrically actuated device 421 S1 may provide a liftand holding action and remains powered to hold the brake about itsrespective fulcrum 450 in the released state. Electrically actuateddevice 424 S2 provides a lifting action and is optionally de-poweredafter as discussed above, therefore becoming redundant. In some casesthe electrically actuated device 424 S2 remains engaged, but it may bepreferable to de-power it for several reasons: firstly, providingcontinuous power leads to the supply circuit dissipating more heat andhaving to consume more power; secondly, the stroke on the electricallyactuated device 424 S2 may be too small to provide any additionalholding action; thirdly, a “dangerous failure” may occur if the actuatorrod becomes jammed while holding the brake in a released condition. Thesecond electrically actuated device 424 S2 is positioned with anactuator rod near the end of stroke to provide maximum force but has acorrespondingly short operating travel. The second electrically actuateddevice, 424 S2, is provided only when required to deal with the extralifting and/or releasing force required to initially kick start therelease of the output shaft as discussed above. In some cases, theelectrically actuated device 424 S2 is positioned with an actuator rodnear the end of stroke to provide maximum force but has acorrespondingly short travel of operation. As discussed above, a secondelectrically actuated device is not needed to provide a lifting actionfor the secondary mechanical brake since the secondary electricallyactuated device is in the “armed” condition when the primary mechanicaltakes up the load of the output shaft.

B. Turning output shaft OFF, i.e. loading the mechanical brake. When thepower is turned off, the supply to the electrically actuated device(s)of the primary brake 421, 424 and the electrically actuated device 421 bS3 of the secondary brake is removed. The respective elongated plates ofthe secondary and primary brakes 410, 415 consequently drop and engagewith the brake shaft to the “armed” position, i.e. ready to be loaded bythe weight of the output shaft. The arming action may be effected by twomeans: a) by gravity whereby the mass of the elongated plates andactuator rods are free to fall and, optionally, b) one or two resilientmembers on the primary brake and the secondary brake provide additionalengagement force.

The resilient members of the primary and secondary brakes are optional.The number of resilient member(s) provided with the electricallyactuated devices (S1, S2 or S3) serve to enhance the brake engagementphase. They provide an increased accelerating force to the elongateplates when being “armed” into contact with the output shaft, andconsequently reduce the time taken for the output shaft “drop” into theengagement with the output shaft, i.e. armed position. They also assistin the mechanical brake to properly nest against the output shaft at thearming stage.

To ensure that the power to the output shaft is not lost before thebrakes have had time to engage, the power to the output shaft is removedafter a short delay (approximately between 0 and 1 second, preferablyapproximately 0.25 seconds) introduced by a capacitive circuit in theelectrical drive unit that controls the power to the motor. Ideally, themechanical brakes is still engaged with the output shaft whilst there isstill power to the output shaft. Preferably, the time from when power tothe electrically actuated device holding the elongated plate in the“off” braking condition is removed and the time the mechanical brakeengages with the output shaft is short (preferably between approximately0 and 0.1 seconds, more preferably approximately 0.02 seconds). Oncepower to the output shaft is removed, the mechanical brake becomesloaded (in the “on” brake condition), supporting the weight of theoutput shaft. At this stage, the primary mechanical brake takes the loadof the output shaft whilst the secondary mechanical brake remains“armed”. As the mechanical brake becomes loaded, there is a normaloutput shaft drop or slip of 0.1-0.5 mm (with no added weight) toapproximately 0.5 mm with maximum added weight. The variance is due tothe mechanical stiffness of the brake and the position of the outputshaft in the stroke within the coil assembly of the motor since theelectromagnetic forces acting on the output shaft vary with positionwithin the coil assembly.

C. Dangerous failure. If the primary brake fails to engage the shaft andprovide braking action, (e.g. for reasons discussed above) the secondarybrake will now provide the braking action, i.e. moving from the “armed”condition to taking up the load of the output shaft.

In the case where the power to the output shaft is turned off as aresult of a safe speed monitor (SSM) intervention, the arming time ofthe brake is seen as the drop of the output shaft since the output shaftis now subject to free fall under gravity (due to less frictionallosses) until the brake is engaged. The presence of the biasing springsminimise the arming time and, consequently, the output shaft dropdistance. The acceleration of the elongated plate and thus, themechanical brake into engagement with the output shaft is dependent onthe resilience of one or more resilient members biasing the elongatedplates into the armed position. In the case where the resilient memberis a spring, the acceleration of the elongated plate and thus, themechanical brake into engagement with the output shaft is dependent onthe spring constant (N/m). In one example, the primary mechanical brakemay comprise a first resilient member producing a force of 10 N and asecond resilient member producing a force of 5 N. The secondary brakemay comprise a resilient member producing a force of 19 N. Theaccumulative forces of the resilient members of the primary brake andthe secondary brake causes the elongated plates to accelerate fasterthan when only one resilient member of the primary brake acted on theelongated plate. The combination of the resilient members of the primarybrake and the second brake reduces the arming time of the mechanicalbrake by approximately 30% in comparison to if there were no resilientmembers, i.e. when the respective elongated plates free fall undergravity.

However, having a dual mechanical braking system should the primarymechanical brake fail can be impractical due to the dimensionalconstraints imposed by the primary and secondary mechanical brake withinthe testing apparatus, in particular the length of the output shaft. Theoutput shaft would have to be made longer to accommodate both theprimary mechanical brake and the secondary mechanical brake, which wouldmake the testing machine too long. Increasing the length of the outputshaft has a tendency to reduce the flexural rigidity of the outputshaft. To provide a more compact braking system, preferably thesecondary brake is rotationally offset from the primary brake about theoutput shaft axis 403. This has the benefit that the primary andsecondary brake engages with the output shaft at differentcircumferential positions on the output shaft. Therefore, the elongatedplates of the respective primary and secondary brakes pivot about theirrespective axes that are offset from each other, i.e. they arenon-parallel. By rotationally offsetting the primary brake and thesecondary brake, the physical dimensions of the secondary brake, inparticular the elongated plate of the secondary brake can be made muchsmaller than the primary brake. For example, the secondary brake can doaway with the insert which fits in the hole of the elongate plate. Inthe particular embodiment of the present invention as shown FIG. 9D, thesecondary brake is orientated substantially perpendicular to the primarybrake about the longitudinal axis 403 of the output shaft such thattheir respective pivot axes 490, 491 are substantially perpendicular.

As with the primary brake, the secondary brake may similarly comprise anoverload stop 454 so as to prevent excessive loads being applied to thesecondary brake mechanism. The overload stop 454 of the secondary brakecan either be mounted to the elongated plate 415 of the secondary brakeor alternatively as shown in FIG. 9F mounted or anchored onto the lowersurface of the elongated plate 410 of the primary brake and comprises asurface that is arranged to contact the lower surface 404 of theelongated plate of the secondary brake when excessive loads are appliedto the secondary brake mechanism. For the purpose of explanation, theelongated plate of the primary mechanical brake can be referenced as apivotally mounted first plate 410 and the elongated plate of thesecondary mechanical brake can be referenced as a pivotally mountedsecond plate 415. In the particular embodiment of the present inventionas shown in FIG. 9F, the overload stop 454 has as an upside down “T” orup tac shape whereby one end of the overload stop 454 is anchored to thelower surface of the pivotally mounted first plate 410 (elongated plateof the primary brake) such that it is suspended from the pivotallymounted first plate 410 (elongated plate of the primary brake) and theother end provides a surface for contacting the lower surface of thepivotally mounted second plate 415 (elongated plate of the secondarybrake). Until contact is made, there remains a gap or spacing 448between the lower surface of the pivotally mounted second plate 415(elongated plate of the secondary plate) and the surface of the overloadstop 454. As with the primary brake, the height and thus, the spacing ofthe overload stop is adjustable.

The overload stop 454 of the secondary brake works in conjunction with aresiliently loaded (or supported) fulcrum 450 b to prevent excessiveloads being applied to the secondary brake mechanism. As with thepivotally mounted first plate 410 (elongated plate of the primarybrake), the pivotally mounted second plate 415 (elongate plate of thesecondary brake) tilts about a fulcrum 450 b that is resiliently loadedsuch that the braking force is controlled by the loading of a resilientmember 451 b. As with the primary mechanical brake, the fulcrum 450 b ofthe secondary mechanical brake can be resiliently loaded or supported bymounting the fulcrum 450 b on a resilient member such as a spring asshown in FIG. 9A. Alternatively, the fulcrum can be resilientlysupported by being mounted to an adjacent elongated plate, i.e. to thepivotally mounted first plate 410 (elongated plate of the primary brake)rather than being separately mounted. The resiliently loaded (orsupported) fulcrum 450 b of the secondary brake comprises a bracket 455that is suspended from an underside of the pivotally mounted first plate410 (elongated plate of the primary mechanical brake) and is biased upagainst the first end of the pivotally mounted first plate 410(elongated plate of the primary brake) by the resilient member 451 b. Aswith the resiliently loaded (or supported) fulcrum of the primary brake450 shown in FIG. 9A, should excess force be applied in a downwarddirection after the secondary mechanical brake has been actuated, thepivotally mounted second plate 415 (elongated plate of the secondarybrake) can be pulled downwards against the force of the resilient member451 b until the lower surface of the pivotally mounted second plate 415(elongated plate of the secondary brake) contacts the overload stop 454whereupon the pivotally mounted second plate 415 (elongated plate of thesecondary brake) will tend to pivot about the overload stop 454 againstthe resilience of the resilient member 451 b so as to permit movement ofthe brake shaft and/or output shaft through the bore/hole of thepivotally mounted second plate 415. Should the excess force be removed,the braking action of the secondary brake will be immediately restoredand the brake shaft held in its new position.

Unlike the resiliently loaded (or supported) fulcrum of the primarybrake whereby the fulcrum is mounted on a resilient member, the fulcrum450 b of the resiliently loaded (or supported) fulcrum of the secondarybrake is not mounted directly on a resilient member. In the particularembodiment shown in FIG. 9F, the resilient member 451 b is located onone arm of the bracket 455 and the fulcrum 450 b is mounted to theanother arm of the bracket 455. The bracket 455 adopts a “C” shapeconfiguration, i.e. a post and an upper and a lower arm. The resilientmember 451 b is located at the upper portion of one arm of the bracket455 and the fulcrum 450 b is mounted to the lower portion of the otherarm of the bracket 455. The bracket 455 is mounted or suspended from theunderside of the pivotally mounted first plate 410 (elongated plate ofthe primary brake) and comprises a spring loaded support post having oneend mounted to the underside of the elongated plate and a free end forsupporting the resilient member 451 b, more specifically the free end ofthe support post comprises a stop for supporting the resilient member451 b. The resilient member is located along the support post betweenthe upper portion of the arm of the bracket and the stop of the supportpost. The stop, as shown in FIG. 9F, may be a plate substantiallyperpendicular to the rod.

The resilience of the resilient member supporting the fulcrum of boththe primary and secondary brakes may be pre-loaded to a value, forexample 650 N, which is above the maximum expecting payload. Theexpected payload is the sum of the output shaft and grip mass, whichwould typically be in the region of 100 N, but may be between 10 to 1000N. Overload protection to protect excessive loads being applied to thebrake mechanism may be called upon in the following scenarios:

A) The scenario whereby the power to the output shaft is switched offwhen travelling at a high speed, for example above approximately 0.5 m/sin the downwards direction and whilst in high power operating mode. Inhigh power mode, the brake is not required to prevent output shaft drop,but it will still provide a retardation braking effect.

B) The scenario whereby the power to the output shaft is switched offfrom high power mode whilst holding a high tension load on a polymerspecimen. In this case the strain energy in the specimen will cause thebrake to become loaded above the pre-load value and the output shaftwill pull through until the stored load is reduced to a particularvalue, for example, 600 N.

During operation of the overload protection, when the brake is in thenormal “on” condition and holding the weight of the output shaft, thereis small gap 449 between the lower surface of the elongated plate 410,415 and the overload stop 452, 453, 454 of the primary brake mechanismor the secondary brake mechanism. This may be adjustable, and mayoptionally be adjusted to approximately 1 mm, although it may be fixedto any value between 0.1 to 10 mm. When the braking load exceeds thepre-set value of the resilient member of either the primary brakemechanism or the second brake mechanism, the fulcrum will move downwardsas the elongated plate pivots or tilts about their respective overloadstop 452, 453, 454, and consequently translate the entire elongatedplate 410, 415 downwards. In this state, the mechanical brake is nolonger self-energising and the limit of the braking force has beenreached. If the output shaft load is increased further, the mechanicalbrake will slip (i.e. the output shaft will slip through binding forceof the mechanical brake) at a constant load depending on theco-efficient of friction of the brake surfaces, e.g., brake shoes(insert)/brake shaft. For example, this could be 700 Newtons. Once theexcess load is removed from the output shaft, the resilient member ofthe resiliently loaded (or supported) fulcrum automatically returns themechanical brake to the normal state.

As discussed above, the limit stops 430, 430 b are employed to preventthe elongated plate of the mechanical brake from tilting in the “off”condition and thereby, ensures that there is clearance between the edgeof the bore of the secondary elongate plate and the brake shaft/outputshaft so permitting the brake shaft and/or the output shaft to travelfreely through the bore and/or the hole. In respect of the primarybrake, the limit stop 430 is fixed to the frame of the test apparatusand is arranged to cooperate with a complementary shaped projectionportion of the primary brake, more specifically the insert of theprimary brake. In the particular embodiment shown in FIG. 9A, theprojection portion adopts a frusto-conical shape. A similar arrangementis employed in the secondary mechanical brake. In a particularembodiment of the present invention, as shown in FIG. 9F, the elongatedplate 415 of the secondary mechanical brake comprises a projectingportion 435 that cooperates with a second limit stop 430 b; the firstlimit stop 430 being the limit stop of the primary mechanical brake. Thesecond limit stop 430 b may be positioned on the underside of thepivotally mounted first plate 410 (elongated plate of the primarymechanical brake). In the particular embodiment shown in FIG. 9F, theprojecting portion 435 of the secondary mechanical brake has afrusto-conical shape that is arranged to be received in a complementaryprofile of the second limit stop 430 b. By means of the cooperationbetween the secondary mechanical brake and the primary mechanical brake,the secondary mechanical brake is allowed to possess a similar functionto the primary mechanical brake, i.e. limit stops 430, 430 b to preventthe mechanical brake from engaging with the output shaft in the “off”condition and the resiliently loaded (or supported) fulcrum 450, 450 bin combination with the overload stop 452, 453, 454 to provide overloadprotection of the secondary mechanical brake when in the “on” condition.

Any of the features of the primary mechanical brake can be applied tothe secondary mechanical brake. For example, additionally, oroptionally, the actuator road of the electrically actuated device 421 bof the secondary mechanical brake can be coupled to the elongated plate415 by an actuator rod extendable feature to allow the actuator rod ofthe secondary mechanical brake to retract further into its housing orsolenoid when the projection portion 435 of the elongated plate 415nests up against the limit stop 430 b in the “off” condition, andthereby, either preventing the secondary mechanical brake rubbingagainst output shaft when in the “off” condition as well as providingmaximum holding force for holding the elongated plate 415 of thesecondary mechanical brake in the “off” condition.

Each option presented in any of the aspects above can be combined anyother option of any aspect of the present invention unless specificallydisclosed as alternatives.

Further Features

A. A method for controlling a braking system of an electromagneticmotor, the electromagnetic motor having a moveable output shaft,comprising the steps of:

-   -   receiving a velocity signal and/or an acceleration signal based        on movement of the output shaft, said velocity signal and/or        acceleration signal having a respective frequency spectrum;

identifying an event from the velocity and/or the acceleration signalusing the respective frequency spectrum, wherein said event correspondsto an uncontrolled movement of the output shaft and has a characteristicfrequency spectrum.

B. The method of feature A, wherein identifying the event comprisesfiltering the velocity and/or acceleration signal to attenuate one ormore frequency components of the frequency spectrum.

C. The method of feature B, wherein the one or more frequency componentsattenuated by the filter represents a part or a whole of a frequencyprofile of the uncontrolled movement of the output shaft that does notpose a risk to an operator.

D. The method of feature C, wherein uncontrolled movement of the outputshaft up to a predetermined movement threshold corresponds to theuncontrolled movement of the output shaft that does not pose a risk tothe operator.

E. The method of feature D, wherein the predetermined movement thresholdis in the range 0 mm to 200 mm or 0 mm to 60 mm.

F. The method of any of the preceding features A to E, wherein thevelocity signal and/or acceleration signal corresponding to the movementof the output shaft is determined from at least one of the following:

-   -   i) a displacement detector,    -   ii) a velocity detector, or    -   iii) an acceleration detector.

G. The method of any features B to F, wherein filtering the frequencyspectrum comprises directing the velocity signal and/or accelerationsignal through a finite impulse response low-pass filter.

H. The method of any features B to G, wherein identifying said eventcomprises

-   -   comparing a filtered velocity signal and/or filtered        acceleration signal with a predetermined velocity threshold        and/or acceleration threshold.

I. The method of feature H, wherein the predetermined velocity thresholdis in the range 0 mm/s to 100 mm/s for linear movement and/or in therange 0 deg/s to 360 deg/s for rotary movement.

J. The method of feature I, wherein the predetermined velocity thresholdis about 10 mm/s for linear movement and/or 30 deg/s for rotarymovement.

K. The method of feature H, wherein the predetermined accelerationthreshold is in the range 5 mm/s² to 30 mm/s² for linear movement and/orin the range 0 deg/s² to 500 deg/s² for rotary movement.

L. The method of feature K, wherein the predetermined accelerationthreshold is about 30 mm/s² for linear movement and/or 90 deg/s² forrotary movement.

M. The method of any of the preceding features A to L, wherein uponidentifying said event, arresting the output shaft, comprising the stepof applying an electrical braking effect using the coil assembly and/oractuating a mechanical brake.

N. The method of feature M, wherein arresting the output shaft involvesactuating a solid state relay switch.

O. A control system for controlling the braking system of aelectromagnetic motor having a moveable output shaft, using the methodof any of the preceding features A to N, comprising:

-   -   a safe speed monitor (SSM) comprising a filter.

P. The control system of feature O, wherein the SSM comprises acomparator for comparing a velocity and/or acceleration of the outputshaft with a predetermined velocity and/or acceleration threshold.

Q. The control system of feature O or P, wherein the SSM comprises adecimator for reducing the sample rate of an input signal, the inputsignal comprising the velocity signal and/or acceleration signal.

R. The control system of any of the features O to Q, comprising a SafeTorque OFF (STO) device for actuating a brake system to arrest theoutput shaft.

S. A test apparatus comprising:

the control system of any of the features O to R;

an electromagnetic motor having a moveable output shaft; and

a braking system for arresting the output shaft.

T. A method for preventing motion of an output shaft, of anelectromagnetic motor comprising a coil assembly, when a mechanicalbrake is released, comprising the steps of:

-   -   determining the position of the output shaft;

determining a current based on the position of the output shaft thatwhen applied in the coil assembly induces a force on the output shaft toprevent motion of the output shaft when the mechanical brake isreleased; and

-   -   applying the current to the coil assembly.

U. The method of feature T, further comprising using a look-up table todetermine the current applied to the coil assembly based on the positionof the output shaft.

V. The method of feature T or U, wherein the position of the outputshaft is determined by a graduated scale attached to the output shaftthat cooperates with an encoder.

W. A method for generating a look-up table for correlating a current tobe applied to a coil assembly of linear electromagnetic motor with aposition of an output shaft of the linear electromagnetic motor so as toprevent movement of the output shaft when a mechanical brake isreleased, comprising the steps of:

-   -   determining a first current needed to hold the output shaft in a        first position;    -   determining, a second current needed to hold the output shaft in        a second position;    -   storing the first and second positions together with the first        current and second current in the look-up table.

X. The method of feature W, wherein the difference between the firstposition and the second position is less than 4 mm or less than 3 mm orless than 2 mm.

Y. The method of feature W or X, comprising the step of:

a) displacing the output shaft axially;

b) sampling the current of the coil assembly at fixed intervals of thedisplacement of the output shaft.

Z. A braking system for controlling displacement of a linearelectromagnetic motor having a linearly moveable output shaft, using themethod of any of the features T to Y, comprising:

-   -   a mechanical brake,

a controller for receiving the position of the output shaft andconfigured to apply an electrical current to the coil assembly toprevent motion of the output shaft when the mechanical brake isreleased.

AA. The braking system of feature Z, wherein the mechanical brakecomprises a resilient member for damping the release of the brake fromthe output shaft such that an impulse, experienced by the output shaft,generated by the release of the brake is decreased.

AB. A method for monitoring the performance of a mechanical brake for alinear electromagnetic motor, the linear motor having a linearlymoveable output shaft, comprising monitoring travel of the output shaftover the duration of actuation of the mechanical brake and comparingsaid travel with a predetermined travel threshold.

AC. The method of feature AB, wherein monitoring travel of themechanical brake comprises:

detecting a first position corresponding to the position of the outputshaft when the mechanical brake is initially actuated such that themechanical brake is engaged with the output shaft;

-   -   detecting a second position corresponding to the position of the        output shaft at a predetermined time or when the output shaft        comes to rest after the first position is detected;    -   determining the travel by calculating the distance between the        first position and the second position.

AD. The method of feature AB or AC, comprising the step of:

-   -   storing the travel.

AE. The method of any of the preceding features AB to AD, wherein if thetravel exceeds the predetermined travel threshold, comprising:

-   -   alerting a user to the degree of travel.

AF. The method of any of the features AC to AE, wherein detecting thefirst position is triggered by an optical sensor detecting that themechanical brake is engaged with the output shaft.

AG. The method of any of the preceding features AB to AF, wherein thepredetermined time is between 0.1 s and 1 s.

AH. The method of feature AG, wherein the predetermined time is 0.25 s.

AI. The method of any of the preceding features AB to AH, comprising thestep of:

-   -   counting the number of actuations of the mechanical brake.

AJ. The method of feature AI, comprising alerting a user of the numberof actuations of the mechanical brake.

AK. The method of feature AI or AJ, comprising alerting a user when thenumber of actuations of the mechanical brake exceeds a predeterminedthreshold.

AL. The method of any of the features AB to AK, comprising the step of:

-   -   communicating the travel and/or number of actuations to a third        party for monitoring.

AM. The method of any preceding feature AB to AL, wherein thepredetermined travel threshold is between 0 mm and 10 mm.

AN. The method of feature AM, wherein the predetermined travel thresholdis about 2 mm.

AO. A mechanical brake for arresting movement of the output shaft of alinear electric motor, comprising:

a pivotally mounted plate having a space for receiving the output shaftof the motor;

an electrically operated holding device contacting a free end of theplate and arranged to hold the plate in a condition to permit movementof the output shaft and to permit the plate to pivot to a jammingposition;

wherein the electrically operated holding device comprises a solenoid tocontrol the movement of the plate.

AP. The mechanical brake of feature AO, wherein the solenoid is a linearsolenoid.

AQ. The mechanical brake of feature AP, wherein the solenoid acts on arod, the rod freely coupled to the free end of the plate, wherein therod is vertically moveable.

AR. The mechanical brake of any of the features AO to AQ, wherein therod is coupled to the free end of the plate by a coupling device, saidcoupling device comprises a resiliently extendable member coupled to thefree end of the actuator rod such that in the extended position of theresiliently extendable member, the actuator rod retracts further intothe solenoid than in a non-extended position of the resilientlyextendible member.

AS. The mechanical brake of feature AR, wherein the resilientlyextendable member is a pre-loaded tension spring.

AT. The mechanical brake of feature AR or AS, wherein the resilientlyextendable member is extendable in the range between 0.1 mm and 10 mm orbetween 3 mm and 4 mm.

AU. The mechanical brake of feature of any of the features AR to AT,wherein the resiliently extendable member comprises an insert moveablewithin a flanged housing and biased in the extended position.

AV. The mechanical brake of any preceding features AO to AU, wherein theelectrically operated holding device is arranged to hold the plate in acondition to permit movement of the output shaft while electrical poweris applied to the electrically operated holding device.

AW. The mechanical brake of feature AV, wherein the plate contacts alimit stop to permit movement of the output shaft.

AX. The mechanical brake of feature AW, wherein the pivotally mountedplate comprises a projection portion that cooperates with the limitstop.

AY. The mechanical brake of feature AX, wherein the projection portionis a frusto-conically shaped projection receivable in the limit stop.

AZ. The mechanical brake of any preceding features AO to AY, wherein theelectrically operated holding device is arranged to permit the plate topivot to a jamming position in the absence of electrical power appliedto the electrically operated holding device.

BA. The mechanical brake of any preceding features AO to AZ, wherein themechanical brake comprises a resilient member arranged to bias the platetowards the jamming position.

BB. The mechanical brake of feature BA, wherein the resilient membercomprises a spring.

BC. The mechanical brake of feature BB, wherein the spring is acompression or extension or tension spring.

BD. The mechanical brake of any of the features BA to BC, wherein theresilient member is coupled to the plate.

BE. The mechanical brake of feature BD, wherein the resilient member iscoupled to the free end of the plate.

BF. The mechanical brake of any feature AO to BE, wherein the mechanicalbrake comprises a stop spaced apart from a lower surface of the plateand arranged to contact the lower surface of the plate when a forceacting on the plate by the output shaft exceeds a threshold.

BG. The mechanical brake of feature BF, wherein the plate is pivotallymounted on a resiliently loaded fulcrum comprising a fulcrum supportedby a resilient member, said resiliently loaded fulcrum is capable ofmovement when the force acting on the plate by the output shaft exceedsthe threshold such that the plate pivots about the stop and overcomesthe biasing force of the resilient member and the plate is held to allowmovement of the output shaft.

BH. The mechanical brake of feature BF or BG, wherein the spacingbetween the stop and the lower surface of the plate is adjustable.

BI. The mechanical brake of any of the features AO to BH, wherein theelectrically operated holding device further comprises an electricallyoperated lifting device for contacting the free end of the plate andarranged to lift the plate in the condition to permit movement of theoutput shaft.

BJ. The mechanical brake of feature BI, wherein the electricallyoperated lifting device for lifting the plate comprises a solenoid,wherein the solenoid acts on a rod to releasably engage or couple withthe free end of the plate.

BK. The mechanical brake of any of the features AO to BJ, wherein themechanical brake is a primary mechanical brake and wherein themechanical brake further comprises a secondary mechanical brake, saidsecondary mechanical brake is the mechanical brake as defined in any ofthe features AK to BF.

BL. The mechanical brake of feature BK, wherein the primary mechanicalbrake is rotationally offset of the secondary mechanical brake about thelongitudinal axis of the output shaft such that their respective platespivots about non-parallel axes.

BM. The mechanical brake of feature BK or BL, wherein the respectiveplates of the primary mechanical brake and the secondary mechanicalbrake are arranged to pivot independently in a sequence orsimultaneously.

BN. The mechanical brake of any of the features BK to BM, wherein theprimary mechanical brake comprises a pivotally mounted first plate andthe secondary mechanical brake comprises a pivotally mounted secondplate, each of the pivotally mounted first and second plate comprise aspace that are co-axial for receiving the output shaft of the motor,wherein the secondary mechanical brake cooperates with the primarymechanical brake to provide a second stop spaced apart from a lowersurface of the pivotally mounted second plate and arranged to contactthe lower surface of the pivotally mounted second plate when a forceacting on the pivotally mounted second plate by the output shaft exceedsa threshold.

BO. The mechanical brake of feature BN, wherein the second stop ismounted to the pivotally mounted first plate.

BP. The mechanical brake of feature BN or BO, wherein the secondarymechanical brake cooperates with the primary mechanical brake to providea second resiliently supported fulcrum, said second resiliently loadedfulcrum is capable of movement when the force acting on the second plateby the output shaft exceeds the threshold such that the second platepivots about the second stop and overcomes the biasing force of theresilient member of the second resiliently loaded fulcrum.

BQ. The mechanical brake of feature BP, wherein the second resilientlyloaded fulcrum is resiliently mounted to the pivotally mounted firstplate.

BR. The mechanical brake of any of the features BN to BQ, wherein thepivotally mounted second plate contacts a second limit stop positionedon the underside of the pivotally mounted first plate when the secondarymechanical brake is not engaged with the output shaft.

BS. The mechanical brake of feature BR, wherein pivotally mounted secondplate comprises a projection portion that cooperates with the secondlimit stop.

BT. The mechanical brake of any of the features BK to BS, wherein theprimary and secondary mechanical brakes are engageable with the outputshaft simultaneously.

BU. The mechanical brake of feature BT, wherein engagement of the outputshaft by the secondary mechanical brake is released prior to the primarymechanical brake by a predetermined amount of time.

BV. The mechanical brake of any of the features BK to BU, wherein thepivotally mounted second plate is vertically offset from the pivotallymounted first plate.

BW. A device for arresting an output shaft of an electromagnetic motor,said output shaft being moveable, comprising:

-   -   a coil assembly circuit comprising a plurality of separate coil        loops configured to cause movement of an output shaft of the        linear motor while electrical power is applied;    -   a switching device configured to form an electrical connection        between the plurality of separate coil loops of the coil        assembly circuit such that movement of the output shaft is        arrested; and

an opto-isolator for actuating the switching device.

BX. The device of feature BW, wherein the opto-isolator comprises an LEDand a photovoltaic cell.

BY. The device of feature BW or BX, wherein the switching devicecomprises at least one back to back MOSFET device electrically coupledto each of the separate coil loops such that in use, under normaloperation, coil loop separation is maintained so as to prevent currentflow through the MOSFET device.

BZ. The device of any feature BW to BY, wherein the device comprises aTVS diode for protecting the MOSFET device from the current exceeding athreshold.

CA. A test apparatus comprising:

a control system of any feature O to R;

a electromagnetic motor having a moveable output shaft; and

at least one of:

a mechanical brake for arresting the output shaft according to any ofthe features AO to BV;

a device for arresting the output shaft according to any of the featuresBW to BZ; or

a braking system for controlling displacement of the linear motoraccording to feature Z or AA.

CB. The test apparatus of feature CA, wherein the motor is arranged in avertical and/or horizontal orientation.

What is claimed is:
 1. A method for controlling a braking system of aelectromagnetic motor, the electromagnetic motor having a moveableoutput shaft, comprising the steps of: receiving a velocity signaland/or an acceleration signal based on movement of the output shaft,said velocity signal and/or acceleration signal having a respectivefrequency spectrum; identifying an event from the velocity and/or theacceleration signal using the respective frequency spectrum, whereinsaid event corresponds to an uncontrolled movement of the output shaftand has a characteristic frequency spectrum.
 2. The method of claim 1,wherein identifying the event comprises filtering the velocity and/oracceleration signal to attenuate one or more frequency components of thefrequency spectrum.
 3. The method of claim 2, wherein the one or morefrequency components attenuated by the filter represents a part or awhole of a frequency profile of the uncontrolled movement of the outputshaft that does not pose a risk to an operator.
 4. The method of claim3, wherein uncontrolled movement of the output shaft up to apredetermined movement threshold corresponds to the uncontrolledmovement of the output shaft that do not pose a risk to the operator. 5.The method of claim 4, wherein the predetermined movement threshold isin the range 0 mm to 200 mm or 0 mm to 60 mm.
 6. The method of claim 2,wherein the velocity signal and/or acceleration signal corresponding tothe movement of the output shaft is determined from at least one of thefollowing: i) a displacement detector, ii) a velocity detector, or iii)an acceleration detector.
 7. The method of claim 2, wherein filteringthe frequency spectrum comprises directing the velocity signal and/oracceleration signal through a finite impulse response low-pass filter.8. The method of claim 2, wherein identifying said event comprisescomparing a filtered velocity signal and/or filtered acceleration signalwith a predetermined velocity threshold and/or acceleration threshold.9. The method of claim 8, wherein the predetermined velocity thresholdis in the range 0 mm/s to 100 mm/s for linear movement and/or in therange 0 deg/s to 360 deg/s for rotary movement.
 10. The method of claim9, wherein the predetermined velocity threshold is about 10 mm/s forlinear movement and/or 30 deg/s for rotary movement.
 11. The method ofclaim 8, wherein the predetermined acceleration threshold is in therange 5 mm/s² to 30 mm/s² for linear movement and/or in the range 0deg/s² to 500 deg/s² for rotary movement.
 12. The method of claim 11,wherein the predetermined acceleration threshold is about 30 mm/s² forlinear movement and/or 90 deg/s² for rotary movement.
 13. The method ofclaim 1, wherein upon identifying said event, arresting the outputshaft, comprising the step of applying an electrical braking effectusing the coil assembly and/or actuating a mechanical brake.
 14. Themethod of claim 13, wherein arresting the output shaft involvesactuating a solid state relay switch.
 15. A control system forcontrolling the braking system of an electromagnetic motor having amoveable output shaft, comprising: a safe speed monitor comprising afilter; wherein the control system is configured to carry out the stepsof: receiving a velocity signal and/or an acceleration signal based onmovement of the output shaft, said velocity signal and/or accelerationsignal having a respective frequency spectrum; identifying an event fromthe velocity and/or the acceleration signal using the respectivefrequency spectrum, wherein said event corresponds to an uncontrolledmovement of the output shaft and has a characteristic frequencyspectrum.
 16. The control system of claim 15, wherein the safe speedmonitor comprises a comparator for comparing a velocity and/oracceleration of the output shaft with a predetermined velocity and/oracceleration threshold.
 17. The control system of claim 15, wherein thesafe speed monitor comprises a decimator for reducing the sample rate ofan input signal, the input signal comprising the velocity signal and/oracceleration signal.
 18. The control system of claim 15, comprising asafe torque off device for actuating a brake system to arrest the outputshaft.
 19. A test apparatus comprising: the control system of claim 15;an electromagnetic motor having a moveable output shaft; and a brakingsystem for arresting the output shaft.